This is one of the reasons why I charge a flat monthly fee rather than charging by the day. That way, I get paid the same each month from all my clients regardless of my vacation time or stats. The only time the rate would change is if I go over the amount of holidays that I have allotted myself for the year, in which case I would deduct that from their next monthly fee (though I don't foresee that ever happening).
I charge a monthly rate too as it works best for me