If it is second focal plane, read your owners manual to find what zoom power your reticle reads true. Then, there are 2 ranging formulas:
Known target size (inches) ÷ Target size (MOA) x 95.47 = Range yds
Known target size (Inches) ÷ Target size (mils) x 27.77 = Range yds
BUT...go try this with lots of targets and double check with a laser range finder. It's most definitely not as easy as just following the formulas. That reticle measurement is easy to screw up. In fact, if you try it 5 times on the same object, you are very likely to get 5 different ranges.