The armscye depth is the measurement I initially found the most tricky to get my head around. For one, I am quite certain I had never heard this term until I started this blog (I don't think it was mentioned once in the four years I studied), but since then, it seems to pop up all the time (always the way, right?) It also seems that there are many different ways people suggest to find it.
When I first drafted my bodice block, this is the technique I found online and used to measure my armscye depth. I placed three fingers under my arm and then measured down from my last finger to my waist. I don't really know what the direct correlation is between this measurement and the armscye depth, but apparently, due to the ratio of the body, these two measurements are the same.
The more obvious way of finding this distance is to measure down from the nape of the neck to the (imaginary) horizontal line that runs across your back, between the bottom of your armholes.