This is where percentage based tipping falls apart. Either he did a $7 job, or he didn't. It was worth $7 last month, why not this month? Why not give him an extra dollar since you got free money, and that makes it 2 bills; easy.
I give better tips at Dennys percentage wise than I do at good restaurants. They're still there busting their asses, so I tip what I think it's worth, or at least somewhere close.