Basic maths:
10 000ft to lose in 40 miles, gives you 250 ft per NM wich is a 2,5 ° angle.
Now depending on your G/S but let's assume today we have 420 kts Ground Speed...that makes 7 nm per minute (420 kt/ 60 =7 nm/min)....you multiply your G/S per your angle (2,5°) 7 x 2,5 = 1750 ft/mn
You can do the same with the mach, if you descent for example at mach 0.7 :
7 x 2,5° = 1750 ft/mn
That's what I use when I 'm bored...