7 hours at 250 degrees = 1750 units
1750 units / 275 degrees = 6.36 hours
7 hours at 225 = 1575
1525 / 275 = 5.72 hours
So between 5.72 hours and 6.36 hours at 275 assuming that's a 7 hour rib cook.
YMWV - close as you can get.
That avatar should be reduced.
