Think carefully about how to notify the school by voice. Say that you over estimated work hours because you calculated based on a year. So, you made a reasonable, but incomplete calculation. Tell them how thrilled you are to be accepted and will accept with enthusiasm and ask how the incorrect estimate of work hours will affect acceptance. I agree that the incorrect hours will have little impact in the scheme of things. That is better than the school happening on the discrepancy. Depending on your commentary, they at already have found out and thought it was irrelevant to the admissions decision. Either way, it will be a relief to no longer worry and if the worst happens, you have other alternatives.