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Pupil @6mm gathers ~28.27mm^2
Scope/EP combo delivers ~162.86mm^2
You are getting ~17.35% of the available light into your eye.
Your 17.5" has area of ~240.53"^2
times 17.35% is 41.73"^2
which is equivallent to about a 7.3" scope with an EP that produces a 6mm exit pupil
You could refine the above by taking into account your secondary mirror size.
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