I hear that, scaling issues have nearly destroyed my War Machine build over and over again. The lessons I've learned the hard way:
1. Just because you need to scale up your bicep by 14%, doesn't mean the rest of the suit needs 14 % (yeah, I did that

)
2. Find a reference point/area in the Pep and use the point to point measurement. Unfortunately for circumferences (cod!) I had to carefully click from point to point around the circle, then add them up.
3. Build in paper first, then again in foam, the extra time is worth it for the cleaner product that fits the first time. I have 2 thighs, 2 cods, a boot, and a shoulder that would all fit an NBA player because of this issue.
It is really frustrating to have to rebuild parts that are too big or too small. My VERY limited experience has taught me more about what I don't know than it has about how to scale stuff. I'm subscribing here in hopes that one of the old pro's tips will help me out too.
Good luck