For me, I charge a wee bit more for p/t clients as they're taking up a f/t space. I don't fill up that spot with another client (incase of an emergency and they need more days).

My daily rate is $40/day (that's for 5 days/week), and my part-time rate is $42/day (2-4 days/week). Depending on your area, you might be able to charge a little more for p/t rates!

Hopefully that helps you out a bit!