Keep in mind that it also takes a while for the effect to dissipate, so short flights or staying in the pattern may not be enough to eliminate all fouling. This means that it may slowly get worse even though you are "flying" the airplane. Also, 30% is about the limit for normal operation, 50% is unflyable, and 60% is backfiring and severe degradation. Ten percent should be noticeable during the magneto tests, and audibly different (in the 114, and after the next update to all my reciprocating engine aircraft).