Thanks for sharing! I can surely attest that the process is tedious!

Though you're working with straightedges, I have a few tips for people making their own surface plates in
how to make your own spherical comparator/ spherometer at:
Workshop Metrology - Meccano Gallery
but also how I made my own "Cheap-o-meter" at:
Repeat Reading Gage - DIYSwede's "Cheap-O-Meter"

Cheers
Johan