Machinist level question

[Sheldon]

Some advise to place these on top of a carpenters level and use it that way. I've done this with a lot of success. Placing evenly spaced razor blades underneath the carpenter level allows you to get the reading for varying widths.

This is really only helpful when leveling the frame. Not as accurate as you would think, and is not really necessary once one gains experience with the machinist level.