That's about how I figure out my rush orders, on a "how much time are they needing this in" sort of way. Right now for example I have a girl ordering place cards for her wedding, on my listing it states that I need the confirmed information at least 3 weeks before the wedding. She was unable to make that happen and I am only going to be getting her info a week from her wedding. I am spending my weekend working on her order, so I charged her a rush fee accordingly and probably would have made it less had I had 2 weeks to work with, etc. On my listing I just have "Rush orders are available, please contact for pricing" That way I can figure out what works best for that order, since all custom orders are different!