Good luck with searching, but instructions are unlikely as the Preciray pre-dates the internet.
To work properly, the rollers should be positioned to match the intended position of the rim, and not taken from an un-true wheel.
The radial roller position is fairly simple as the height of the edge of the rim above the bead seat doesn't vary much, but the lateral roller needs to allow for the width of the rim as well as the hub OLN dimension. For example, if the hub OLN is 130 mm, and the outside rim width is 26 mm, the edge of the roller should be (130+26)/2 = 93 mm from the face of the wheel clamp opposite the roller, or (130-26)/2 = 52 mm from the roller-side wheel clamp face. Hopefully the scales that aren't properly visible by the name plate and at the bottom will help with this.
I'd suggest putting a known good wheel in, and seeing what the various scales seem to show.
Brief notes from someone who's used one here