I have just been going by judging the lump method. My snake is still relatively new, so he doesn't come out much during the day, but he had no lump after about 18 hours with 1 pinkie, so I bumped him up to 2 pinkies, and he had a lump for about 48 hours, no regurg, no hassles, so that is probably about perfect for him.
According to what I read, the 2 pinkies thing is great for 5 or 6 feedings before moving to fuzzies, and by then you should have a pretty good feel for your snake, and be more able to tell when he is hungry, or needs bigger prey.