1. Bert's got 20 years on me.
2. A standard film holder backing plate is .197 .007 from the film holder edge for formats 2x3 through 4x5.
http://home.earthlink.net/~eahoo/page8/filmhold.html
3. A Graflex film holder backing plate is .224 from the film holder edge in 4x5, tolerance unknown.
4. The view hood ground glass can be shimmed up to 1/8 inch (.125) higher without causing problems, lowering more than .010 inch may require milling the mount opening down.
5. TriX TXP sheet film is .007 inch thick.