[GroundUrMast]

I don't have a neat and tidy formula to offer. The factors are going to be unique to each situation.

Some of the key factors to consider would be;

Are you facing co-channel or adjacent channel interference?
How directional is the antenna?
What is the actual overload point of the amplifier?
Will the antenna be aimed at a high power signal when aimed at the lower power signal.

Some of these factors can be quantified easily while a consumer may be left to experiment to find the answer for others.

Filters are common in commercial applications where a hundreds or thousands of dollars can be budgeted. There are no over-the-counter single channel bandpass filters in the consumer market that I'm aware of. The closest thing to that would be products from tinlee.com who builds to order. Your alternative is to invest in your own test gear and try you hand at building your own... you'd truly be bitten by 'the bug' to be willing to drop a few thousand dollars into such a hobby interest.