Ahh.! i remember my special straight edge with the notch cut out for the center stand..
The trick is to get it nice and sweet with the straight edge or string line (tedious or what..!)...
.... Then mark each side with a subtle punch mark or scribe, so that subsequent adjustments can be made relative to your marks.
Unfortunatly any adjuster is handicapped by the maths of the set up, as a very tiny amount of movement at the end of the wheel spindle will effect a relativly huge deflection when measured at the front wheel....1mm of adjustment on one side of the spindle will move the center line by about 10mm at the front wheel..