Did you read the May 5 posts on this forum entitled "??How thick should I go??" ?

Some people have made very thin plate glass mirrors. However, grinding a thin blank requires extra precautions to avoid astigmatism in the surface. Moreover, plate glass expands and contracts more readily than pyrex and this makes figuring a plate glass mirror to a F/5 parabola much more difficult.

I know it costs more, but since this is your first mirror, I recommend that you go with a thicker blank made of pyrex.