Celestron secondary mirror for C8 and C11 act like approximately 5X multiplier. So for every millimeter spacing added can add 5mm back focus assuming the calculation is linear.
C925 has longish OTA so I assume the secondary multiplier is less than 5X. Not sure about C14 because native focal ratio is f/11.
Ooh, that checks out, actually, although I think you said it backwards. My backspacing is currently a couple mm too long and my focal length measures out at 2810mm instead of 2800mm. But there is certainly some uncertainty in my measurement. So if I really wanted to get to even f/12, that’s an additional 560 mm of focal length, requiring an additional 112 mm of back spacing, or 4.41”. I could probably do that, but anything higher is probably prohibitive.
The right solution is a Barlow. The problem is that the optical train is currently all M42 back from the Starizona T adapter with filter holder. I’d have to replace the entire optical train to put a Barlow in. After I just spent $65 on the T adapter. :Op
I suppose I could go back to my old optical train, but it was a kludge complete with compression and parfocal rings. I like the all-threaded connections of the new setup.