[ATM] How round and how square should it be (blank mirror)

[Roman Toledo <rtoledo2002@yahoo.com>]

          Hello gents, here's another noob question.

How round does the blank have to be and how square should the side be.

I've read how some mirrors use a sling or piano wires on the bottom
edge once mounted on it's base.

I have one piece that was done without trepanning and is "close" to
being round, I'm working on putting a good 45^  dege on it before I
continue to wet grind the edge (I know at this rate it will take even
longer)

this question even scares me ;) but I'll ask, if you plan on a 9 point
base and a piano wire sling. 

Would it be a good idea to cut a "slight" 1/16"deep groove on the 
glass edge's center so the wire stays put?  or is this too great a risk
to damaging the mirror?

also do you have a good source for  polarized film so I can test this
glass before I waste time on a useless piece.
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