For purposes of "calibration step size" PHD needs to estimate the guider's image resolution and asks for pixel size and focal length of the guide scope. So my question is when using the OAG-8300, is it correct to assume the guider focal lenght is the main scope's focal length affected by the OAG-8300 reducing factor ( 0.7 ) ... ?. My 102mm f/8 refractor is a native 810 mm, so when using the OAG-8300, PHD settings for focal length would be : 810x 0.7=576 mm
That is approximately correct. The exact reduction factor will vary slightly depending on focus position, but this should be close enough for your purposes. If you have MaxIm DL the PinPoint Astrometry command will give the exact focal length.
