Some where between my ES 9mm and 14mm 100 degree eyepieces. Trying to use my Pentax more to justify its purchase though.
But its so difficult to go from 100 degrees of the ES to the 70 degrees of the Pentax and be... satisfied with the view. Especially when sharpness wise there is barely a difference. I mean its definitely there. Just not the incredible and magical views that people here would have you believe.
The 9mm-14mm in 12" dob give magnifications between ~160x and ~110x. Which, thanks to the apparent field are very useable magnifications.
In my AR102 the 10mm Pentax throws up a very nice low powered but very contrasty view at 66x. My 9mm and 14mm all of a sudden become lower powered searcher eyepieces. My ES 30mm 82 degree provides a staggering 3.5 degree view. Superb at cruising through the Milky Way star fields.
The point of all this is that these eyepieces are in a useable range all the time. When you choose your eyepieces. Try for 3 eyepieces that give you exit pupils of ~1mm, 2mm and 5 mm. With a distribution like that you will be set.
In your C9.25 that's no less then:
10mm @ 235x at 1mm.
20mm @ ~110x at 2mm
40MM @ ~60x at 4mm
Explore Scientific have a range 68 degree eyepieces that will cover your longer focal length eyepieces perfectly at a very reasonable price. Alternatively you can go in swinging with a ES 100 degree 20mm for that 2mm exit pupil. It will be without a doubt your most used eyepiece.
Hope I helped.
