[ATM] Optical Flat

[polaraligned at optonline.net (polaraligned)]

Hi,
I am building a 25" f/5 folded newtonian using a 9" flat.  (right angle
folds)
I have been offered a flat with a strehl ratio of .96.  I know this is
phenomenal
for a primary mirror but I have never heard of the strehl ratio being
applied to
a flat.  I have other companies offering lambda/10 flats, for much
less money than the company that offered the strehl .96 mirror.  ($3500 vs.
$1400)
What kind of quality do I need and what kind of paper work should I ask for
from the company that I choose to make the flat?  Can anyone recommend a
company to make me the flat?  Also, would it be wise to have a center hole
in the flat for mounting?


Thank you very much,
Scott