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Help me understand scaling loupes/magnifiers with reticles

I have been working on collets for rectangular/emerald cut stones. I have been using a method in a Hans Meevis tutorial to make something like this (photo is from his tutorial):

The method is performed without a collet block. I do not have a rectangular collet block, and I don’t really want to pay for one since they only accommodate calibrated sizes. My problem is with consistency of 17/34 degree angles. I came across this kind of tool:

It has a “protractor” on one of the reticles. I think it might be helpful to confirm angles but am not sure how it would work for 3D objects. It seems like it would be great for flat objects but how could measurement work on something like I’m making to obtain consistent angles? It is touted everywhere as being good for machinists and jewelers. Has anyone found this type of device useful?