Hi,
I shave my head with a straight.
I always use the same 5/8 round point for my head; I don't change size, grind etc. as I do for my face.

I find skin stretching and a very keep edge essential.

I do use two mirrors but only to check I'm removing everything. For the back it's by feel, a bit like combing your hair (I only do WTG).

The first time I tried I found it easier than I expected; around the ears is the only tricky area.