Be careful asking the FMS and VNAV (Vertical Nav using FMC data) to do too much. Our new guys come out of training having had "use VNAV" drilled into their head by the training department and they have a hard time recognizing when to abandon the technology and revert to old school ways. For descent planning at idle thrust in a Boeing I have always used 100 miles for 30,000 feet and then three miles per 1000 feet for the remainder. Then factor in 1 nm for each 10 knots to slow to 250. For example: I am at 41000 feet and need to cross ABC at 250 knots and 10,000 feet. I am planning a 300 knot descent. I need to lose 31000 feet to cross ABC at 10,000. The extra 1000 feet will require 3 miles and I will need 5 miles to slow from 300 to 250. So 100 + 3 + 5 = 108 miles. Add a mile for each 10 knots of tailwind and you will nail it. It is not perfect but it works well enough for simpletons like me. My pet peeve is when pilots are quick to deploy speedbrakes at the first sign of getting high. Simply lower the nose and increase speed a little and it will accomplish the same thing. The passengers will be rewarded with a smooth decent instead of shaking, rattling and rumbling all the way down. After all your careful planning ATC will surely throw something into the mix to screw it all up.