Most people here measure from the nape because those are the layers that typically show length, but I think if you just pick a spot and stay consistent with it, that should be fine.
I actually measure the sides, nape, and crown when I length check. However, I only post pics of the nape.
(don't ask why I'm up)
ETA: You have beautiful hair!
ETA2: I was on my phone and didn't see the poll. I chose pull down since my hair is natural and has about 50% shrinkage. However, like I stated above, I also use a measuring tape from the nape and have a length check shirt too. So I guess I use three methods.
erplexed I think most ppl here use the pull down method and then a length check shirt, then measuring tape on nape.