Determining when to splash in limited is all about weighing power versus consistency. The better a draft deck is, the more it wants consistency. If a great deck can cast its spells on time, every game, then it will usually end up ahead of an opponent whose deck is generally weaker than ours. The worse a draft deck is, the more it wants power. A bad draft deck can be saved if it gets to cast a few high-impact spells, so the bad deck is more willing to sacrifice some consistency in order to get access to these spells. We need to look at splashing and fixing differently based on how powerful our deck currently is and vary our range accordingly. I think of my drafts in three groups: good decks, average decks, and bad decks.
The Good Deck
A good deck is a deck where I already have multiple powerful cards in a color or colors that seem open, so I can be fairly confident of more powerful cards coming to me in pack three. This is the ideal situation for a draft. At least two thirds of your decks should be in this range if you have strong draft fundamentals. In this range, I make several adjustments. First, I take on-color dual lands higher. My deck is already powerful, so I can pass up a little power to gain consistency. Second, I start avoiding double mana costs, as they often cause midgame mana problems. I particularly try to limit my double costs to only one color, since I want to play a manabase like 9/8, 10/7, or 10/8, and I don't want to try to cast any doubles off of seven or eight sources. Third, and most importantly, I narrow my splash range to only the very best splashable cards in the set. By about my seventh draft of a format, I try to have a set list in my head of which cards those splashable bombs are, that way I know what fixing to favor in case I open them. For Shadows Over Innistrad, I'm still working on the list, but it's something like:
As you can see, the list is very short. Discipline is very important here. Powerful cards are attractive and draw the eye, but as competitive players it's important not to take off-color cards if we think we can win without them. A large consistency increase is more important than a medium power level increase. The best time to take bombs like this is early in pack two in a pack where we are not missing out on a powerful on-color card. Here we can speculate on the bomb because we have ample time to look for the fixing required to maintain our consistency. Even after taking a card on this list, I insist on at least 8/8/3 mana before I will splash it in a good deck, so I tend not to take these cards in pack three if I haven't yet seen fixing for them, and am happy to leave them in my sideboard if the fixing doesn't show.
The Average Deck
Average decks are more of a balancing act. An average two-color deck can still win a draft, but it usually needs to get lucky to do so. This type of deck is in at least one open color, but hasn't opened a lot of powerful spells or had to give up an early pick after being cut out of that pick's color. In this type of deck, I'm now a bit more willing to sacrifice some stability if the upside is making my deck considerably more powerful. Here I prioritize potential splash fixing, favoring lands that are in only one of my colors or nonland fixing that can make or fetch multiple colors. I now lower my mana requirements for splashing the top bombs from the above list to 8/7/3, as this deck needs the power more to win. I then widen the range for splashing with 8/8/3 mana to include premium cards that are not bombs, such as powerful unconditional removal and card advantage generating creatures. I will also consider splashing more than one card in this deck if I can get four sources to do so.
A common scenario for this in Shadows Over Innistrad is the green delirium decks, which can make use of cards like Fork in the Road, and Vessel of Nascency to easily splash either black or white off of a single basic land. When in this deck, if my power level is not looking high enough, I will start to aggressively take potential splash cards like Kindly Stranger or Bound by Moonsilver since there are enough common and uncommon enablers for splashing to play them with a good manabase. This is an easy way to make an average deck powerful enough to win the draft if we balance the fixing and the power correctly. The key is identifying that our power is a bit low and opening our range up to include these strong off-color cards.
The Bad Deck
When our draft is going very poorly, there are a few paths to avoid a trainwreck. If my deck is leaning toward a high creature count and a low mana curve, I will simply focus on consistency. I won't splash at all, I will simply take vanilla creatures and combat tricks and hope to punish my opponents for stumbling or playing greedy decks. This type of deck is more likely to get there by running well in terms of mana and curve than by casting one or two extra removal spells. If my deck is leaning toward the slower and clunkier, I then open up my range as much as possible. I will now splash the top bombs in the format with no fixing at all and simply run a bad manabase. I will now splash less efficient removal spells and worse creatures if they fill important holes in my deck's strategy. I will more liberally splash multiple cards or multiple colors, even off of not quite enough sources. The key idea here is that a bad deck will never win without getting lucky, but we want to make sure that when we DO get lucky we are doing the most powerful thing available to us. This situation is fairly rare, it should come up in less than 1/10th of your drafts, but when it does, don't let yourself draft a deck that can never win. Shoot for a deck that is either very low variance or very high variance. Try to either punish your opponent's bad luck or get lucky yourself. It may not be a good plan, but it is better than no plan.
A final note: the specified ranges should be your default, but they can be adjusted for specific strategies and matchups. If your deck is very controlling and is certain to get to the lategame in almost every game it plays, you can splash more liberally even in a good deck, because you will be less likely to die with uncastable cards in your hand. Likewise if you see most of the opponent's deck in game one or game two and it seems certain that neither of you will be killing the other quickly, it's often a good idea to sideboard in a greedy splash that you left out of your main deck for the same reason. If you are going to see 2/3s of your deck in game three, don't worry so much about having the mana you need, you will have time to draw it.
The key here is becoming very good at evaluating the power level of your deck during the draft. This requires a good knowledge of the format and the fundamentals of drafting, but once you get to that level, you will be paid off by knowing exactly where to value different kinds of fixing and off-color cards. Knowing how good your deck is and how good it needs to be to win can let you correct your course mid-draft and come out on top even when not everything goes your way.