Here is a formula for you to use:
1.To calculate when to start your descent = 3x Altitude required to lose then remove the zeros
2.To calculate the rate at which you need to descend = 1/2 Groundspeed + 0
Example:
Crossing restriction - Cross the LON VOR at 10,000ft
You are cruising at FL350 and your groundspeed is 400knots.
1. You need to lose 25,000ft (difference between FL350 and 10,000ft) x 3 = 75,000 remove the zeros = 75
So that means start the descent 75nm from the LON VOR
2. 400knots Groundspeed divided by 2 = 200 then you add one 0 = 2000.
So that means your descent rate should be 2000FPM
Hope that helps