Just to make sure you have the correct formula, it is: Height of the target in inches x 95.5 divided by your reading through the scope in MOA (at it's proper magnification for correct ranging) = Range to the target in yards. The fella's have given you formula's (all correct) but if you are just getting started, it can be a bit confusing.
So in the field it would it would look like this: You have a 2/3 scale IPSC piece of steel (20" tall) and it measures 2.3 MOA. So the math is simple: 20 x 95.5 (1910) divided by 2.3 = 830.4 yards. Find a firing solution for 830 yards and send it (of course factoring your wind call). Hope this helps without being confusing.