My pet sitter walks my dogs every day and I pay her very well, so I don't generally tip except at Christmas and when she goes beyond the call of duty. (Like on this trip, I'm not getting home until late in the evening so I threw in extra since she's feeding them dinner.)

If it's a one shot deal, I'd probably tip the amount you suggested just to make sure she'd be willing to make time for me the next time I needed her.