Horizontal Planes

Sorry for the multiple list posting, but it is a question concerning
CNC machines, fabrication, and precision. I have posted a picture that
I drew which should help explain the situation a bit more. I am looking
for a good way to check for flatness of a frame that will be the CNC
table. The image is here, http://webpages.charter.net/aprs/level.JPG
and basically out of all the possible measurements one can make to an
object ,this is the only one you can not use a tape measure on. I
bought an expensive, brand new Starret StaTru bubble level thinking
that would be the ticket to use by checking both ends of the frame,
well, other than the pretty shiny red powder coat finish on the level
it ain't no better than what you could buy at the local hardware store,
sad. So is there any old school method that they maybe used when making
aircraft by hand in 1942 or something?

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