is it just a case of rounding up?
i wouldnt do that ? you want to be telling your cam software exact sizes

with vernier calipers i use the full length of the faces of the calipers to pinch as much of the cutting section of the tool as i can

i guess makes more sense to use a micrometer but that can be a pain as there are peaks and troughs when measuring with such a small surface aria that micrometers have so in that situation i revolve the tool in the mic until it starts to grip (revolve the tool away from the cutting direction)

youll get a feel for it