Five tons per acre doesn’t sound completely out of line, especially with red clay, but I’d be a little cautious about treating that as a fixed rate without a soil test. The CCE of 70% is also worth taking into account since that isn’t the same as applying 100% pure calcium carbonate.

If you can’t get the soil tested before the lime arrives, I’d probably check with whoever supplied the recommendation and ask whether the 5 tons/acre figure was already adjusted for the 70% CCE. It might also be worth getting a sample tested afterward, so you have a better idea of where your pH and alkalinity actually ended up once the pond is filled.

Red clay can vary quite a bit from one site to another, so I’d rather have a test to work from than keep adding lime based on a general rate. Hopefully someone here with a similar clay-bottom lake can give you a more specific comparison.