extra percentual , choose base of calculation

for an extra based on percentual value, it would be very helpful to define
on which value this percent will be calculated.

at the moment, we define for example 4 percent for an extra (insurance for cancelation, percent per room rate).
but we would be able to define, this 4 percent based on final room rate (including discounts).

at the moment it is only for rate room base, without discounts.
so it is not comprehensible for customers at the moment.

thanks
Beni

www.ferienwohnungen-davos.ch