I am interested in drawing some gridlines on my 4x5' ground glass. I would like to know the rectangular dimension of the 4x5' film that actually gets exposed (i.e. area that is not blocked by film holder guides/flap) and how they relate to the orientation and placement on the whole 4x5' film sheet. Does this vary with different film holders? Ie. I would like to know the exact rectangular dimension I should be composing to to include everything on the film.