Helpful Tech Spec (or maybe not).
 

Fon anyone looking to scribe a focus mark on the casework or set up initial backfocus with the standard or modified SLR camera lens mount, front of housing to rear groundglass surface on Letus35 Flip for XL1+ is 40mm on the specimen I am currently modifying for a client.

For a quick check with a tape measure, the front face of the velcroed battery pack is close enough at 2mm forward of where the focus mark would be. (by strange co-incidence, 2mm = the cumulative thickness of the two velcro strips.)

For front face of Nikon mount, measure 6.5mm from face of housing and you should just about have backfocus spot on or at least within usable range for wides.

If you use a 10mm metal or phenolic board to make a surrounding clamp and saddle mount around the front housing for rods support, the battery back must be relocated if you intend to use the rear face of the saddle mount for a focus mark as the saddle will otherwise not sit rearwards far enough.

For the sake of 2mm I am unlikely to bother and instead maybe add a small block and pin to rear of the clamp and saddle mount for the focal plane mark to hang a tape measure off and position the clamp and saddle furthur forward.

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Correction to above. My bad eyesight misread the vernier scale. This specimen is 39mm from front face of body to focal plane. So to find correct flange face for Nikon, measure 7.5mm from front face of body to front of Nikon mount.

Sorry for the hiccup and don't believe everything I spout forth.