How does one determine spacing of lens precisely enough to determine length of a box camera hyperfocal construction?

I can probably rig up a bellows in the front for smaller lenese, or a sliding back.

I have an Aero-Ektar 12" f/2.5 I want to make a box camera with. It does have front & back focus numbers but I don't know if the barrel conversion it's in is correct...with surplus conversions, you never know.