I use the mirror method also. A mechanical level isn't accurate enough, especially for large enlargements from 35mm. The evidence is the enlargers at the local school, where they use levels instead of an optical method (lasers or the two mirrors). First surface mirrors aren't required, most modern glass has the two faces close enough to parallel to not matter, and I find it easy to ignore the extra reflection. I used a piece of heavy copper wire hammered to a point to scratch the silvering off, but any method would work. As others have said, level isn't required, but having the 3 planes parallel to each other is. Using two mirrors to get an endless reflection is close enough for our purposes, and a lot cheaper than the lasers. With the mirror facing down, I put a right angle prism over the hole. This lets me look straight ahead instead of climbing on the table to look down. A normal mirror is placed on the baseboard.