There is a simple method which works reasonably well and as a simple soul even I can work these out in my head.
DESCENT POINT - DIVIDE FLIGHT LEVEL BY 3
So FL300 divided by 3 is 100. So you need to start your descent 100 miles from destination.
Keep checking during the descent, so passing 10,000 ft, 100/3 = 33 miles from touchdown.
RATE OF DESCENT TO MAINTAIN A 3 DEGREE APPROACH- MULTIPLY GROUNDSPEED BY TEN AND HALVE THE RESULT
Groundspeed is 400 KTS. 400*10=4000. 4000/2 = 2000FT per minute Rate of Descent
You need to recalculate as the aircraft slows down in the descent.
So on final approach GS around 120 KTS. 1200/2 gives you a rate of descent rate of 600 FPM.
Hope that helps