I apologize if this sounds rude, but why exactly do you (and others) think the rest of it have it all figured out?
It's not like we have some perfect and super easy way to scale pieces that we don't share with others. So not-even-asking: "my arm is this big i need the scaling ASAP" isn't an answer. (Punctuation and a "please" would go a long way. That word is magic, you know.)
No one's armor is the same. No ones arm is the same. My personal advice is to
research! Look at reference pictures, get an approximate size with Robo's method, build the size tester, (probably more than once unless you've figured out your scale) take a deep breath, and relax. Because nothing about armor making is ASAP.