Quote Originally Posted by Calamity Jane
..the glass must be positioned so that the inner (frosted) surface of the glass is the same distance from the camera back as the film will be. In 4x5s, that's 0.195" or 5 mm (if I remember correctly)...

Here's a page that lists the critical "t" distance you refer to as well as other dimensions for 4x5 and all common sheet film holders: