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Brian,
I use a duo band filter in similar skies and 300secs is the longest I use. Best thing is try it and see if it is enough for your intended target. Mind you I rarely go for really faint targets from my backyard and make these a focus for my dark sky trips. You can always do longer darks if needed. Just do your flats using the same filter on the night and you should be good to go.
Mike
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