Not a problem!
Do the following:
Take the altitude you need to descend (let's say, you're at 3000 feet above ground level) and multiply it by three, then divide by a thousand.
9 is our number, which is the number of miles from the runway, you wanna begin your descent.
Let's say your ground speed is 90 knots, in which case, the full formula would be:
Feet to descend*3/1000=X
Feet to descend/(60/Groundspeed (90)*X)=Y
That should give you 485, which is your desired descent rate in this case, unless I've forgotten something...
