Please don't misinterpret my comments, regarding countertop cutoffs/cutouts. If you have the money and the perfect flatness REALLY matters then buy a certified, inspected new surface plate or a used one that comes from a reliable source with a recent inspection. After that the assurance of flatness goes down hill, and there are no guarantees on used surplus plates. But it probably doesn't matter for the vast majority of us and any used surface plate will do.
Granite slabs are mass produced and surfaced at factories with machines that are going to produce a pretty locally-flat
generated surface.
Check out
this video.
At a countertop shop, they will have a bunch of handheld grinders and polishers used cleanup and fix-up ie. finish countertops for installing. If the chunk you get has been touched by a hand held tool, then yeah it's potentially going to have issues. A sink opening from a factory slab that has simply been cut out is likely to be pretty good and possibly free. (ask for the cutouts if you buy a countertop for your house).
I have a cast iron bench plate that was given to me - free. I consider it the "reference" flat surface for my shop. It's maybe 12" X 16". I've checked for rocking against my 123 blocks and the longest precision straight edge that I have and numerous squares. By no means a conclusive test but it's as flat as the tools I have to work with.
I use my table saw top with abrasive paper on top to lap things and sharpen edges of chisels. For lapping, I check the pieces against each other and the bench plate.
Regarding the original posters question, if you can verify that tube sides are flat and parallel (don't rock on a flat surface and indicate the same height) you can then check the ends against other tube ends for squareness. A little tricky and time consuming but possible.
For what it's worth, you don't need something flat to make something flat. Check out the by-hand glass grinding and polishing of optical flats.
Fundamentally, there are only 2 surfaces that will remain in full contact when rubbed against each other, a plane and sphere. And a plane is just a sphere with in infinite radius. If you grind two pieces of glass against each other you get a concave and convex spherical surface with a very large radius. If you add a third piece and rotate through them, they will work towards flat because convex on convex will wear in the middle, concave on concave will wear on the edges. If you have good contact on the whole surface, switch one out and continue. Keep doing this carefully until you can't tell the difference and you are done!
For glass, you polish the glass when it flat enough and then check the interference lines (from monochromatic light if possible) and continue to polish for flatness - trying to get all the interference lines straight not curved. Because you have 3 comparisons you can use simple algebra to find the difference between any two.
For metal, you could probably use scraping techniques until all 3 pieces indicate even wear / contact.
(sorry, way TMI - I get that way)
