Hi @James--Pad-A-Terre-0
Other hosts with experience in similar cancellation policies will be in a better position to comment on the calculation of the refund regarding the Airbnb commission.
Regarding your decision-making, I think you handled this in the best way to secure a good outcome. The guest did not want to stay, so the most important thing in this situation was to free up the calendar as soon as possible. I hope you manage to get the nights booked again soon. It sounds like the dates are still some time into the future, so hopefully you have enough time to get booked without the need to offer discount. If so, this should make up for any money lost.