How would I go about making a script to position a character's M6D Welds relative to the character's size?

What I want to find out is to see how doing what I have specified earlier would be done, obviously, the answer would be for CFrame math I assume, but the issue with that is I am not the best with CFrame math, and trying to think of how to do it just puzzled me. I haven’t found much of help elsewhere either.

Any help on this matter would be greatly appreciated!