Well...since bacterial growth is exponential, I really don't think its much of a problem for them to "catch up" after the addition of a new fish (within reason...adding a grouper to a tank with 2 neon gobies is obviously not a fair example).
Also, we have no way of knowing what percentage of LR surface area is being utilized by bacterial populations, so assuming 20% may be way off. I honestly don't know...
Chris