Your method interests me.
I get jobs into my workshop where I need to map out bolt patterns on automotive clutch covers and then drill holes in a flywheel to suit.
Can a variable sized circle be superimposed onto the screen in place of the cross-hairs.
My thought is to zoom down onto a bolt hole (.312" - .375" dia.) and adjust the screen circle to match the hole rather than using the cross-hair thinking this may be more accurate.
Did you use a 'off the shelf' web camera and mount on to a shaft?
Are some web cameras better than others for this use?