That is why I said use a level to get the lens stage parallel to the base as the first step. Admittedlya lazer might be better, however the errors introduced by a negative carrier differences and different heights of the head pulling the column forward differing amounts exceed any additional accuracy gained by lazering the neg stage to perfection. Check it out, every carrier is different. Every head height is different.
As long as you are projecting a rectangle instead of a trapezoid, you are sufficiently good.