The dilution method for testing chlorine above 5ppm with an OTO test is wildly innaccurate. 1 to 1 dilution might get you in the ballpark but a 4 or 5 to 1 test I would trust like a used car salesman.

The FAS/DPD method is really the only RELIABLE way to test much above 10ppm. You would probably get closer by waiting 'til Cl goes to 5 or below and calculating what you need to bring it up to a desired level.