but will it also work on the View Cart page where customers can also change quantity? 
yes, that is why I suggested the calculation, the calculation will work on any page where quantity can be edited.
You asked questions very similar to this in another thread as well, and i answered you there. It is best to keep to one thread for the same subject to avoid confusion please.
Where yo are working with custom coding, I really cant offer a lot of support, I can try to explain the theory and give examples, but those examples may need to tweaked as they are theoretical examples only.
It is not that I am trying to "railroad" you into another premiere support appointment.
You are changing the requirements of what the code implemented in premiere support was intended to accomplish. That never works very easily. The code Ray created was specific to the requirements you gave during the appointment, If you had put all the requirements up front such as accounting for inventory, ray would have implemented those in the same appointment, and i'm pretty sure it would not have changed the price.
It is always more difficult to add functionality after the fact than to develop for all of the requirements up front.
