Automatically calculate Total in OrderProduct ?

Hello,

I added some attributes in my OrderProduct object, and I need to change the current calculation of "Total".

At the moment, when setting a new value for "quantity" or "price", it updates automatically my "Total". I need, for some records (depending on a boolean), to calculate the Total with another formula, for instance: Total = Quantity * A, which applies when "quantity" or "A" is modified.

However, for these records, "total" should not be recalculated when modifying "price".

I cannot figure out when these calculation are set.

Thanks !