IMHO:
Checkpoints every couple of miles doesn't make sense to me. As the previous guy pointed out you can't get lost that fast!
An example:
Assume I am marginly competant. I have to fly 20 miles on a heading of 270. I fly 260 because I didn't set the DG properly. According to the 1-in-60 rule after 60 miles I'd be 10 miles off course. But if my checkpoint is at 15 miles I'd be 2.5 miles off course. 2.5 miles is managable. Anything less is hard to notice.
By using a shorter checkpoint distance you'd get writers cramp from writing down all the numbers, risk a midair because you can't be looking outside that much, drift off heading because your focus is inside and fail to detect that you aren't actually on the right course because you didn't wait long enough between checkpoints for drift to be apparent. Bear in mind that your airspeed and heading are as accurate as you are but that the winds aloft forecast may well be wrong. If you devote your time to flying in a straight line and keeping track of time to the nearest minute then any difference between your planned and actual position was caused by winds aloft.
Another thing: There's no way you can choose good checkpoints every couple of miles. By settling for an average of about 17 miles you can pick *good* ones that will give you confidence that you are where you are supposed to be rather than a selection of crappy ones. In poker terms your checkpoints need to be "3 Kings" rather than "a pair of twos".
theRolfe