the infamous question.....
I have read on so much my brain hurts.
IF you have an image of your set up even better. plan is asi 294. with ( and without the zwo OAG (16.5mm)
I have read 80. I have read 55. and everywhere in between.
My friends rc8. we plan to set up at 80mm. take an image. check edegs. add/remove spacers untill we like it. then figure out the degress of FOV. with website and caluate the FL. I figured out the formula. basically 749.66 divided by the FoV in degrees.
and then input that for his guiding etc..