Exactly.

The Regulation has a number of anomalies which deal poorly with payments by other than cash or its equivalent, but it seems that are dealt with fairly nonetheless. E.g. 75% of avios paid returned in avios.

The simplest way to deal with the issue is to wait until your return and make the claim in a clear and succinct manner setting forth the avios & cash charged, your calculation of 75% due, what was reimbursed and what is due. Make a specific "demand" for that sum in a polite but firm manner, e.g., citing the Regulation, and be done with it.

Arguing with BA is wasting your time. If the response is to advise you of BA's policies, simply proceed to CEDR.

In the event that there is a change in taxes, remember that the new taxes, rather than 75% apply. Thus, if downgraded from CW to WTP on an xLHR flight, the new APD would be calculated as well.

To those who ask why BA insists on its formulation, the answer is likely simply that the vast majority of passengers accept what they are told and are happy enough with what they got.