I'm sorry I don't agree with some of the things you have mentioned.
I know some people (some even aren't skinny) that have the elbow as most prominent part etc
And I believe you! There is individual variation, it's not a perfect rule and I should have given that caveat
I'm just repeating what I've learned and witnessed in the field.
Also, wasting as a term would be more aptly used not in this context. That's on me. But it still is a good rule of thumb! See-
Ex. Patient w/monomelic amyotraphy
Patient's right arm afflicted- look at upper arm in relation to elbow again.
That's all! Sorry to thread derail