Your enlarger only has to be level with the plane of the earth if you intend to complete your alignment with leveling devices. This can be exasperating if your leveling indicator has poor visibility in certain positions or is bulky and hard to position in tight spaces.
Your enlarger must be in alignment – not necessarily precisely level. The base and/or easel must be parallel in all planes with the lens and negative stage. This three-way alignment doesn’t require absolute precision, but closeness counts a great deal.
I have found it easier to adjust the negative carrier and base/easel without the lens in place. There are several ways to accomplish this. One quick way using the level idea is to put a sheet of glass in place of the negative carrier at the negative stage, long enough to extend out from the enlarger body sufficiently far as to allow easy placement of your level indicator. Then do what ever is required on your particular enlarger to adjust. The lens stage can be done the same way on some models (remove the lens from the lens board et c.
I have found it easier to us two mirrors. One set upon the base/easel, the other placed on the negative stage in place of the carrier. This mirror must be modified by making a small hole in the silvered backing (1/4 inch or so). Use any knife-like tool to remove the backing – it doesn’t have to look pretty. Position the hole in the approximate center of the stage opening, mirror sides opposing. If one peers through the hole, one sees a repeating image of the inside of the enlarger giving the impression of infinite tunnel. If the two mirror planes are not aligned, the image will appear to bend one way or other. Messing about with the negative stage adjustments or the base should bring the images into proper alignment. Applying one of those office-type hole reinforcing doughnuts around the hole on the mirror side helps.
A big problem with this technique is that it can sometimes be hard to get one’s eye into proper position. Eg., I have to remove the light source from both my enlargers to do this then stand on a step stool balancing … you get the idea. This can be overcome with a third mirror used to see the apparatus from the side at right angles. A prism would be ideal.
Good luck and happy printing,