Forget about spirit levels they aren't precise enough. I learned it the hard way. What worked for me was using two mirrors. One on the baseboard and a second one with a small hole (just a few mm in diameter) fixed to the negative carrier. After that stage has been adjusted fix it to the lensboard or (less reliable) filter ring of the lens.
Peek through the whole from above and you will immediatly understand how this method works. This is second best to a laser-align but virtually free.