I bought a new 2'x3' x 3/8" piece of plate/float glass and its not flat.
Over the 3' there is a gap of .02" checking with a good straightedge. My No
7 plane actually rocks on the glass. The underlying surface is flat. I've
read that all plate glass is now the same as float so I don't think I bought
the wrong type.

I previously had (and cracked) a 1/4" thick piece that didn't vary by more
than .003".

What is a normal tolerance? Does thicker glass have greater tolerance for
error?

-- Mark