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Don't know if this is for visual or AP. For newt astrophotography I started at F5. I would recommend there (or even higher - others might know better). Collimation not too fidgety. But you will need a coma corrector for AP at F5 - I found a cheapish Baader coma corrector was adequate. I had a lot of fun with a GSO 8" F5 and a Canon camera then an asi071 (bother aps size).
I've since gone to F4 but can't recommend it to start with. You are in for a world of pain unless you fork out for a good coma corrector - and a proper collimation tool like a cats eye (it's just the greatest!). The baader CC just didn't cut it at F4 so I swapped to the skywatcher/GPU CC. There's a few threads about this - one by startrek - read it and believe!!
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