a simple trick is to take the wingspan of the model, divide it by the wingspan of the full scale, and then turn that decimal into a fraction. You normally won't come out with an easy fraction, but if you turn the decimal into a percent, then it's easier to get a 1/x fraction. For example, if I get 0.312 I can turn that into 31.2%, then I can round that to 33 1/3% which is 1/3. for some weird numbers, like 1/7 scale some FDP charts make it super easy.