> (tool thinking)
> Suggestions sought. Experience preferred.
Go ahead and use second pyrex disc as a tool. An f/7 curve is so mild that it won't matter at all. You can re-use it later for another mirror easily.
As for tiles, you shouldn't use a SINGLE tile. 1 inch square ones would be just fine (and in fact much better, because gap between the tiles speeds up the flow of grit and helps grinding action). But more about that later, when you're ready for your second mirror. For now, use both pyrex discs and go full steam ahead!
Bratislav