Personally, I just calculate how long I have in minutes in my head (approx) until the fix, then figure how fast I need to come down given the altitude I need to lose or how long I can wait before I have to come down. I usually err on the get there a tad early side just incase I eff up a bit. I fly a saab so these calculations are extremely easy. Usually we descend at Vmo which, with TAS and giving a little fudge factor gets me just under 5 miles/min or, take the 10s of miles to your point and double it. 20 miles - 4 minutes.

This only works with airplanes that are doing about 250 KIAS, I'll then make a small adjustment + or - if there is a huge wind or I am going extra slow or something. I think the biggest key is knowing how long you have to make your descent, after that it is pretty easy.