No easy way unless, as stated, you wing it. It's complicated if fact:
Current Flight Level - Target Flight Level = difference
difference / Decent Rate = Time needed for decent in minutes
Time (/60)in tenths of an hour * IAS = NM needed for Decent
NM for Decent + NM Target from end of RWY = Required Distance for Decent
Example with rough rounding:
21000 FL - 2000 = 19000
19000 - 1200 = 15.833
15.8/60 = 0.264
220 IAS / 0.264 = 58.06 NM
58.06 + 10.1 NM from RWY (last waypt)
= 68.16 NM
I only use it for the higher altitudes to give me a chance at getting to the runway without a nose dive.