formula in a self defined field

formula in a self defined field

I made in the module "orders" the fields: abonnement; salesfee% and salesfee €

I want the following:
Salesfee € = abonnement * salesfee%

Our dealer gets 20% fee of abonnement.
I want to register per customer what the fee in € is.

Who can help me with this to make it work? The answer of Pradash did not help me.

Kindest regards
Sybrand A. de Jager