Not sure why you wanted SAP to calculate and arrive the value differently. In SAP, for all item conditions, system would fetch the value based on the condition record you maintained. If you dont want this value but want to change, why don't you set the condition type as manual condition so that whatever value you want, you can key in manually.
Needless to mention, with the billing user exit, this can very well be achieved, I don't think, it is a Best Practice
G. Lakshmipathi