Apologies, was away from my DM Screen and the ole memory ain't what it used to be. The modifier is 15% (not 20%), per 5pts of the Athletics check.
So, if you get a 31 on your check [20 roll + 5 Str mod + 6 prof bonus], divided by 5 is six increments of 15%; total of [.9 * your 20 Str score]...