Modified Work Arrangement and Overtime

We currently have a Modified Work Arrangement with our employees that allows us to average 160 hours over 4 weeks and work up to 12 hours per day without paying overtime. As we are a construction company who works outside, during the winter we typically only schedule an 8 hour day rather than the 12 that we are permitted under the arrangement. My question is, when we are scheduling the 8 hour time blocks, are we required to pay overtime after 8 hours in a day if the average hours over the 4 weeks is still met or only after 12 hours per day as is indicated in our arrangement?