I try to remember how grateful I was on April 15 that at least I had a little less work because some of the procrastinators were filing extensions.  My fees are based on time and effort needed, and there is more of that involved six months after getting out of the routine of finishing several returns every day.  But if I wanted to add a line item to an itemized bill, how would this work?  If there is a balance due, 2.5% of the amount, since you're saving the client 5% by making sure they're not a day late.  If there is a refund, 3.5% of the amount, since that's how much interest IRS will pay if they don't send the refund until December.  (For credit-elect, that's how much 2210 penalty is avoided.)