Just a suggestion, if I may:
Use a light beam to measure flatness. A light beam is pretty accurate for straightness (very slight curvature due to earth's gravity.) One suggestion is to use one of those laser levels (such as B&D's.)
If one is into playing with optics, one could use interferometry to display the flatness of the table. However, this is obviously an extreme. Who needs to meaure the flatness of a router table to 1/4 wave?
A question: how accurate do you need the top to be?
I have been eyeing some straight-edges. Home Deport in Canada has a Johnson 48"/96" straight-edge for CDN$26.88, while Lowes in Canada has a Swanson 50"/100" straight-edge for CDN$27.98. I would think either of these do for checking the flatness of the table.
By the way, I have been eyeing these straight-edges for the same reason as one of yours: I am currently designing my own router table and will rely on the straight-edge to get the table top flat.
I recently purchased a Bosch 1617EVSPK router for this table. Nice kit, but a bit short on some things. Most notably, there are are no fence and no fence rods. Also, if one uses the countersunk screws to hold the baseplate on, the hole for the through-the-table height adjuster in the baseplate does not align with the adjuster. (This is a mute point for my design, as I intend to build a custom mount into the table -- thereby eliminating the supplied bases..)
Last edited by Cassandra; 01-06-2008 at 11:03 AM.