I would start with just 5 SS for being the first attempt to include it. 5 SS per base sound good. To make it relevant I would say that it should be at last double the size than a 50mm marker (difficult to measure but use criteria, walls for example could have less area in general, but who cares, it should count) or just do not include scatter terrain as their size could be negligible.
I think the final word should be arbitrarily on the organizer, so this option is not abused (just ask at the beginning of the month).