[BillA]

so why on earth are you adding the GPU heat load to the CPU sink/source ?
you need to think on what you are setting up for

cal the shunt at its use temp
running a shunt at over 30% will cause progressively more self-heating -> changing the resistance

you should engage in conversation with Lee Garbutt
use his system as your guide if you do not wish to use professional equipment
a clamp-on meter ! wheeeeee

BTW, this is ALL dangerous, particularly for the uninformed
if you are not comfortable, and proficient, at working with electrical circuits -> DON'T DO SO