simple question: how do you measure valve diameter?

is it the outermost circuference of the valve? (surely that would vary between manufacturers) or is it the outer diameter of the valve seat?

Valve face diameter is done around the margin/knife edge (widest part of the valve). AKA head width.
Generally you will get margin,seat and angle specs from the manufacurer. The head width is not really required to machine the valves. Only time Ive measured the head width is to ID what cylinder heads I was looking at.