Dinol
Make sure you remove all the extenders on the guide camera. My 290MM Mini had both the long and short extenders stacked on it when I first got it which made it impossible to achieve focus.
For the first rough focus, I set the OTA on a table in the house looking at as distant a target outside in daylight hours that some fine detail feature (trim/louver/high line wire, etc.) that I could use to focus the OTA on with its autofocus routine that used contrast.. Yes, the prism depth in the OAG is the rough focus adjustment. If you shove the guide camera all the way down and still not in focus, loosen the grup screw and move the prism in a MM and rinse repeat. I had to do this a couple of iterations before I was able to find the guide camera was within adjustment range.
I had the OAG-L focus adjustment set at midpoint. My goal with the prism depth was to leave around 1mm or slightly more 'gap' between the guide camera and the bottom of the OAG-L camera well to provide some additional space for course adjustment if it became necessary. I used the find adjustment to get as good a visual adjustment on the landscape feature I was targeting as possible. That puts the OAG-L/Guide camera in a good starting position for the final "tweak" on a star at night.
(FYI, I add an IR 400nm filter to the guide camera to mitigate 'flickering' at night).
For the final 'tweak", set up the OTA and have it tracking on a star near the celestial equiator and meridian (this to minimize the thickness of the atmosphere/seeing effects). Run the autofocus routing on the imaging train, then put a batinov mask on the OTA and switch your viewing to the OAG. I have to manual slew the scope to get a bright star positioned in the OAG FoV so that good spikes are visible from the batinov mask. Use the OAG-L micrometer adjustment to get the batinov patter centered and lock it in place. Remove the mask and go about your imaging.
The only thing I dis the OAG-L/ZWO on is that the prism shaft grub lock screw is behind the tilt plate and thus inaccessible unless you dismount everything, make the adjustment, and reassemble and iterate the focus test. I don't have one, but from what I've read, this is not an issue with the QHY OAGs. I came so close to dumping my ZWO setup after my bad experience/bad quality of the ZWO standard OAG. So the lock screw location being my only issue with the OAG-L is a big accomplishment.
YMMV.
