Kevin, that helps. What I'm shooting for is to be able to fill the entire film sheet with a given subject. So if life size is three inches long, how much bellows and at what distance to get to nine inches or 1:3. So for a 210 mm lens I'll be at 620 mm of bellows, for 300 mm I'll be at 900 mm of bellows. Just knowing this helps me greatly.

I am focusing on the glass, but I'm thinking that it would be helpful to know how close to set the lens to the subject initially. Working inside moving the camera around is not that difficult. However, if I decide to go out into the field with my 8x10 cambo legend thats a lot to contend with. But, if I get a bit of experience inside I should be able to get my initial setup pretty close when I'm working outside. It would still be nice to calculate it though.

So as for a calibrated focus target do you mean something like this :http://www.salzgeber.at/disc/