...Or you can do what my instructor taught me - forget all the hard maths involved in trying to figure out the distance at which you should start your descent and then figuring out where that is - just do it by time.
You have an ETA calculated for your leg, and say 500fpm is a comfortable descent, you want to lose 2000 ft, so 4 minutes before your ETA set up a 500 fpm descent. No hard maths, and no need to figure out how far - or where 4 minutes at 110 knots is.
Dave