John Aniano in Central NJ
Wiley,

You're spot on! But, I'd recommended to Jeremy to get as thick a piece of glass as possible, 1/2" or thicker if you can find it. Glass is an almost perfectly elastic material and 1/4" thick stuff bends quite easily. Try it! Look at the reflection and bend it, it easily moves. 1/2" bends too, but much less. If you want a really flat lapping substrate surface, go with a granite plate - they are usually 2" or more thick.

John