Hi Matt,
It really depends on your goals, not the answer you wanted
For example, if you want better resolution, you would be less forgiving on high FWHM data and the rejection rate would be higher. It also depends on how mach data you have and are likely to collect on the target, you might need to keep more subs to get the S/N up. If its a target that particularly dim, you will need to keep more data.
Image scale also plays a part, there is not much point in being particularly picky by discarding high FWHM data on a relatively wide field target or where you are grossly under sampling.
Josh