which module is for calculation?

I need to modify the way noC calculates products. A percentage of products my client sells are priced and calculated as following:

item A  (unit per foot)

item A will be used for door installation so I need to let customer to enter door-size. Usually door size is something like
12 1/4 x 27 2/3

I will place door size in a formula and will be multiplied by its price (per foot).

questions:

1) How can I access the door size in code if it is defined as attribute (text box)?

2) which module should I work on?

Thanks.

Hi,

I am trying to do a similar thing, have you had any luck with this?

Thanks!