They can exceed 15% for specific things if expressly authorized but generally, no, they cap at 15% per and 25% total.
Calculation is based on 15% of disposable income.
So 30* fed min wage per week (217.50) is "safe" then they take 15% of the remainder. You can go through a hearing process but generally the hearing officer or ombudsmen are pretty strict, ie HBO is not a legitimate expense. If you go through it bring copies of literally every bill and receipt and be organized, you don't want to go through it unprepared.