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.