Mo,
If you look at fig 2 on this link you can see how the action of injected rapid insulin is very different from that of natural insulin .
http://www.uspharmacist.com/content/s/126/c/20874
Endogenous insulin has already done it's job by 2 hours so it's a different scenario for someone who isn't relying on injected insulin. At 2 hours rapid insulin is only just beyond it's peak action, as Brett pointed out you don't want to be too low at 2 hours because the bolus insulin still has a way to go.
If you started from a lowish level and it's back to pre meal at 2 hours you could be hypo by 3 .
The target I was given when I was first diagnosed was around 50mg/dl (2.7mmol/l) above the starting level at 2 hours. I certainly don't always manage it.
How high you go with any given meal, is obviously dependent upon where you started (and whether you've recently exercised). The content of the meal (ie type of carb, amount of fat etc ) also influences how quickly glucose levels rise and how long they stay risen for. I find that higher fat tends to result in longer, later, rises for example and that for a very low carb meal I have to give a higher insulin dose than I would if I just took the carbs into account.
(I think sometimes it's a bit of an art rather than a science, you get to know that you might need more or less than the nominal carb count for certain meals For others you might inject a bit earlier (as Nigel mentioned) and sometimes you might even split the dose)
