Car fuel costs or mileage allowance

I am a self-employed psychotherapist in secondary occupation and work in a group practice, here I travel by car. I also make other work-related trips by car, but these are less ‘fixed’.

In the learning center, I read: “For travel related to your secondary occupation, we return to the rules for self-employed persons in their main occupation. You take into account the actual costs, but you may only claim the percentage related to your secondary occupation.”