If the reduction of 2% in your payments to her cover her liability to you in terms of costs then it seems sensible to deal with it in that way. You should notify your ex in writing that this is what you are going to do and you should also explain the reason for it. You should keep a copy of this letter because you may need to refer to it later.
That is because, strictly speaking, you need to make a court application in respect of payment of the costs. If you pay less to your ex in lieu of the costs then she may take action to recover the full amount from you. If she does that you would then need to counter claim for your costs. You would refer to your letter on the issue of who should pay the costs of these various applications.
The way you propose dealing with it makes sense but you cannot be sure that your ex will respond rationally. Her previous application to the court is evidence of that. You do need to try to be squeaky clean in case she tries something similar again.
