Setting the neck straight - the camera angle not quite in line to my eye angle here - but the principle is to support the instrument on
the glass plate with the underedge belly level. Blocks and shims for that. The square is then aligned precisely to the centre tail pin.
The far string support wedge moves side to side. When eyed down to centre lower edge - the string needs to line up with centre nut -
adjust the line for that - then centre mark fingerboard and middle ff's should all visually line up. The neck is dry fitted in the mortice
and clamped up. Carefully removing the clamp, if the joint is good it will stay put. The scroll centre should also line up - but the centre nut is more important as the scroll cut can wander of a touch. This system works even if the neck is set at a 1 deg lean to the treble side - which can visually throw things off.