I don´t think this has been proven but from my experience every 10% is a jump in the scores.
10%, 20%, 30% hits hard last time I saw it.
Anyways, the lower the better. I wish someone here could come with a number though
I can't speak to the 30% but I know from experience over the past several months that 20% is a marker for me. At or above 20% and I lose about 6-10 points from being at 19% or below. This may vary also depending on what else is in your credit report i.e. what "scoring bucket" you happen to be in. FICO has so many moving parts it is oftentimes difficult to extrapolate a minor change in score being directly related to an action in your reports.
I do know that a new negative would be a drastic change. And I am looking forward to 02/13 to see if the last negative information to drop is a large positive. I know currently there is a 60-70 point difference between my EQ (clean) and my TU (dirty with 1 negative).