A little side note we have found is in using 6 qt tubs we get about a day shift either way depending on the number of eggs in the tub. 7 - 8 eggs in a tub normally go faster and 5-6 is the normal. Anything below 4 tends to take an extra day.
I think this is because as the eggs get closer to hatching they start some decomposition. This produces heat and with a lot of eggs in the tub it adds up.