I have encountered this before with nurses/cops/firefighters etc. You could charge FT fees, as you can't fill rotating days in between. I do my fee schedules one month at a time, so I get their schedule and see if they use at least each of the weekdays even once in the month, if so, then it's definitely FT fees. If they use less, let's say 4 out of the 5 days once, then I charge for those 4 days every week in the month(ie. mon-thurs will be charged for all weeks-at a higher part time rate of course) 3 day a week minimum, but you will lose $ to let them flip flop like that. I've learned, and now they pay FT or have set days they pay for each week of the month. Or the other option, is to do a 3 day a week minimum and charge $10-15 higher a day and call it a "drop-in" rate.