Your utilization is based on the amount that is reported by the CC to the CR, and most CCs report on the statement date. So let's say your card's statement drops on the 5th. You charged $800 throughout the previous month but paid off all but $100 by the 3rd or 4th. Your statement will show a balance of $100, which is what is reported to the CR, and will translate to 10% util for FICO scoring. Now if you goofed and forgot to pay before your statement dropped, you'd be stuck with a much higher util reporting (and probably a drop in your score) for the next month until your next statement date. Make sense? The good thing is that once you get your util back in line, you'll recover any drop in your score.
