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I did check it at work on a granite layout table with a dial indicator in our machine shop. It was within a few 10,000ths of an inch. Just how good do you think you needs it to be?

I think the...

A couple of points:

1)The floor tile is lapped with a diamond tool. It is not as smooth as glass, but is probably about as flat.It will wear smooth right away.

2) When you lap, you randomize...

Uh.... A floor tile, not the floor. I'm married and not crazy. At least where I live, a granite tile is a lot cheaper than a piece of 1/4" thick glass.

These are common grits for making telescope mirrors. You can get them at gotgrit.com

I bought the 500 SiC for about $2.75 / quarter pound (way more than you need).

You don't even need glass (see my earlier thread). I used a granite floor tile and it worked just fine.
I don't think the grit size is especially critical either - I used 220 SiC and 500 SiC. What...