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Well, to be strictly correct, the bandpass shift is proportional to the recessional velocity which is more or less corellated with distance.
I would expect 3nm filters to be fine out to 30M light years, give or take, without much loss, but at 42Mly the Halpha line is actually shifted to 658.3nm (bang in the middle of a typical Nii filter transission curve, centred on the strong Nii line)
Around this point however, the weaker 654.8nm Nii line will be shifted to 656.8, so you will record it through the Halpha filter... somewhat ironic, hey?
If you are using a faster focal ratio telescope, the higher angle of incidence in the optical train will compound the issue so you will experience a higher reduction in the emission line signal.
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