Here's my two penn'orth
Plate glass, as in shop window or glass display shelving is ground and polished till optically flat. I have a slab about 3/8" thick which sits atop a thick baize wad on top of a slab of chipboard worktop. With the tools I have I cannot detect any area which is not perfectly flat, it therefore suits me far beyond the accuracy I wish to obtain. I use another similar set up for a lapping plate. If you drop something on a steel or cast surface plate, it needs scraping, do the same on plate glass, and it is probably in the bin! I have a close friend, ex career precision engineer, who used to make templates for a granite counter top company, and also inspect the finished work Their granite was ground flat and polished to "optical standards" as it was possible to detect defects in reflections from the granite, which used to annoy some of the wealthy clients as there seems to be a race on amongst the mega rich to have the largest single piece of granite worktop in their kitchen! Cheaper granite worktop is ground flat, but then "mop" polished which puts on a beautiful shine, but doesn't guarantee "flatness" You could of course use some bluing or lapping compound to check a piece of granite with plate glass to see if you got an even, all over "touch" but for the purposes of HM I think we are as near perfect as makes no difference. Don't sweat it, git er done.
Plate glass, as in shop window or glass display shelving is ground and polished till optically flat. I have a slab about 3/8" thick which sits atop a thick baize wad on top of a slab of chipboard worktop. With the tools I have I cannot detect any area which is not perfectly flat, it therefore suits me far beyond the accuracy I wish to obtain. I use another similar set up for a lapping plate. If you drop something on a steel or cast surface plate, it needs scraping, do the same on plate glass, and it is probably in the bin! I have a close friend, ex career precision engineer, who used to make templates for a granite counter top company, and also inspect the finished work Their granite was ground flat and polished to "optical standards" as it was possible to detect defects in reflections from the granite, which used to annoy some of the wealthy clients as there seems to be a race on amongst the mega rich to have the largest single piece of granite worktop in their kitchen! Cheaper granite worktop is ground flat, but then "mop" polished which puts on a beautiful shine, but doesn't guarantee "flatness" You could of course use some bluing or lapping compound to check a piece of granite with plate glass to see if you got an even, all over "touch" but for the purposes of HM I think we are as near perfect as makes no difference. Don't sweat it, git er done.