22K & 15c is not a reduction. There is not thrust reduction until above 30c = ISA +15.
Indeed there are authorities that do not allow derates; everything is FULL Power e.g. 26K and then assumed T. I'd always assumed that the power setting calculated had to satisfy the take off profile parameters. It could be derived in many ways. 26K 44c should be within a gnats cock of 24K 36c. Both should be the minimum required to satisfy the legal requirements. I assume and have the faith.