I need to know how to measure the oak frame pieces for my finished stained glass. The oak frame stock is 2". How far from the panel itself should I measure?

I don't have problems with Zinc framing but have never done wood. I bought the oak wood stock but don't know how to measure to get the miters right. You state "Remember to add for the mitre allowance to get the proper outside size to cut the frame." What do you mean. I know there has to be some type of formula, I believe I got the 2" stock. Do you measure your piece and add 4 inches and then delete what the routed inside measurement is? HELP!!!!