F = 206.265 x Object size in pixels x CCD pixel size in microns
Object size in arc-seconds
So it turns out that with the DBK + 2.5xGSO barlow I get Saturn to appear approximately 80 pixels in diameter, which combined with the DBK's 5.6 micron sensor and Saturn's 17.7" diameter gives out: 5220mm. Dividing 235mm of primary mirror into that, I get f/22.2. Comparing this data with my January 14th images of Saturn which were done using the 2x Ultima + DBK, I get as-near-as-makes-no-difference, 79 pixels width for Saturn & 17.5" which gives f/22.1!
I guess the fact that my focuser has about 20mm of travel does play a role on just how big the targets will appear on the chip and maybe, the 2.5x isn't exactly 2.5x but a little less. I have to measure exactly what the effective f/ of the 9,25 is at 0 extra travel all the way to 20 and take measurements with both barlows to come to a conclusion on what is the exact f/ I'm using.
As for Saturn, here's a view under good but not great seeing. There's more coming.