A couple of days ago I got the leveling feet I ordered, I got them installed and started leveling the base.

Having never used a precision level before, I just plunked it down on the base and started adjusting the leveling feet. This particular model of level has 2 bubble vials, a large vial on the long axis of the level, and a much smaller vial across the body of the level. I EVENTUALLY realized that you need to get the bubble centered in the cross via BEFORE you attempt to center the bubble in the main vial. It makes a BIG difference in how that bubble reacts. That's why you see the level sitting of a piece of paper in the above photo.
Last night I went around the base several times with the precision level, it's as level as it's going to get. I thought that I had leveled the base of the workbench when I installed it almost 20 years ago, if so - I did a really crappy job of it. The base of the machine is about 400mm x 600mm. from the NW corner to the SE corner of the base there's a 6-8mm difference in length of the adjusting stud. The workbench top is flat, it's just not level. Eventually I got the base close enough to level that the paper shim was no longer needed to have the cross vial's bubble be centered, I could just set the level directly on the base.
Next I check the to see that the frame is level, and then it'll be on to the X axis rails.