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Well...
it doesn't appear as though you are including the distance from the secondary to the eyepiece, where the light actually comes to focus???
You should also look to not exceed the maximum useable magnification based on aperture of your scope.
Most people work of 50x per inch of aperture, in very good seeing conditions.
I'd play it conservative and work off 40x.
So, that'd give you 180x ... max magnification.
But I'd say a 6mm eyepiece is about as hard as you might want to push it, in very good conditions. (150x... based on 900mm focal length)
That should help for a start.
others will chime in shortly
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