Hi Sarah,
You can set
Fixed Prices for distance sectors in Address search - Taxi Style, because Fixed rates you are referring to are for Fixed rates between 2 pre-defined destinations where the distance is not taken in consideration.
To set up Flat rate for distance sectors please go to Car management > Edit car > Distance sectors > Add New > Distance From: 0, Distance To: 40, Price structure: Flat rate, Price: 80.
Repeat this for all of your distance sectors and Save.
This is it, now prices will be Flat rate for those Distance sectors.
To disable Price calculation debugging go to back end Taxi Booking > Settings > General Price Settings > Debug price calculation: No.
Note: These rows with price calculation are only visible to Admins, regular users will not see them even when logged in to the booking form.
Hope this helps.
Kind regards.