add a low water mark at 90% of variable TL3 requirements

As imagined here:
https://meta.discourse.org/t/user-rank-should-be-a-little-more-forgiving-and-give-more-insight/17759/14?u=mcwumbly

Adds a low water mark so that trust level 3 isn't lost unless certain settings dip below 90% of the required minimums