no just use comon sence. when you take your j arms off cut them in half ans use that diameter tube.(i dont even know if Jarms have tubes or not, never seen one) but i would say some where inbetween 1/8 and 3/32 wall thinkness should be good. its gonna be fun making thoses things though, lol, j/k. $140 is expensive but it might have bushings and baring fittings that you will have to make or buy. never took my a-arms off to see what there all about but i know mine leak greese.lol set-up a good jig on a table. your gonna have to put a (fishmouth) or what ever its called to get those to buttt close. to make upper and lower a arms fron scratch, figuring out your own dimentions is gonna take maybe 8+ hours.
i would but them and mod them to fit on my bike. but i dont know how much you get paid an hour and if you like your job at all.
if i hated my job i would probably make them no matter how long it took me, lol.
good luck man