I can't count the number of times I've heard the line "I can't do math and ride at the same time". If you are riding possibles with a rollchart, then there really isn't any math involved (well, not much anyways). In a nutshell, riding possibles means you ride up to about a tenth of a mile before a possible check, wait until you are in the correct minute for that possible check, then go like stink until a tenth or so before the next possible.
For instance, if the rollchart said:
MPH = 24
44 8.6
45 9.0
46 9.4
then you would ride until about 8.5 (less if there was a good spot to hide a check, or you had less faith in your mileage or the courses mileage), wait until your clock just ticks into minute 44, then ride your ass off until 8.9. Lather, rinse, repeat.
Remember, a sneaky trailboss can always come up with a way to make it difficult on the people that are blindly following a computer. How? There are lots of ways actually, but here's an example. Suppose I've got 8/10's of a mile of nasty cr*p thru a miserable rockpile that I know people won't be able to average more than 12 MPH in. If there is an easy high speed section leading to the rockpile, then I could switch everyone to 23 MPH at 1.5 miles before the rockpile. The guys riding possibles would know that there wasn't a possible check for 2.3 miles, would fly thru the easy section, and possibly make up for the time they are going to lose in the rocks (for this example, they would need to average 45 MPH). The guys just following their computers would cruise the easy section at 23 MPH, and end up being 2 minutes down at the exit check.
Anyways, I've probably made this sound more complicated than it really is. I'd highly recommend Tim's timekeeping class that he gives at outings from time to time.