Just to let you know, in figuring out the thigh armor sizing for my own Mk VI build, I built half a dozen different sizes of low def thighs (after building two high def thighs and failing) before I found the right size. So, building multiple sizes to figure out the right scale isn't uncommon. You can use all the mathematical formulas you want, make a detailed diagram of your body, whatever; sometimes a scale, no matter how perfectly calculated, will just have to be adjusted.
Where possible, look for low definition files that have the same rough dimensions of the high def files you wish to build. They're easier/quicker to build, so if it's not the right size, you can easily adjust and start again. When you find that size, you can move on confidently to the higher def file. Keep in mind that low def files do not exist for all armor pieces; but like I said, where you can, use size testers.