Here's a simple rule of thumb I use for monitoring descent profile in a 767. It would probably work well in the 757 too. Here goes . .
Take your indicated airspeed and subtract 200. Divide the answer by 10. For example, for a 300 knot descent, the "number" is 10.
Multiply your altitude to lose by 3, add the "number" and you will get a very accurate required track miles for any given altitude. It also compensates automatically for winds and ties in nicely with the "gates" that most people use, eg. 5000ft/20nm/250 KIAS and 10000ft/40nm/300 KIAS.
So next time you are descending through 11500 ft AAL at 280 KIAS and wondering how many track miles you need, using this rule of thumb you will come up with about 43 track miles.
WG.