Optimal FB use involves minimizing the number of provinces getting peasant regrowth (favors fb on few provinces) as well as maximizing peasants fb'd (favors fbing the highest peasant province, aka many targets). In addition, you want to leave some provinces high peasants to facilitate chaining.

In general, you want to fb one at a time to some target level. The target level depends on how many provinces you can chain in the near future and therefore want high peasants and also how many fireballs you can cast. I think 10% peasants might be too many fb's to be optimal typically for a first run of fb's. Another factor is that you may want to have different fireballers target different wpa targets and therefore need more than one target at a time (aka a high and a low wpa target).