How do I set a price per square foot in a calculator pricing engine?

Question: I would like my customers to specify how many banners they would like to order, and how big those banners should be. I then want to charge per square foot. How do I set this up in my calculator pricing engine?
You'll want to have three Quantity Parameters:
Q1: Number of banners
Q2: Length in feet
Q3: Height in feet
Then, you'll also have at least one Calculator Parameter. You can name this parameter whatever you'd like, as it will be hidden from the customer. Once you generate the Excel file, here are the basic settings you will give to that Calculator Parameter:
Per Thousand Weight: the weight of 1000 square feet of your banners (if you're not using integrated shipping, this entry isn't technically necessary)
Show As Option: No
Applies To = Q1*Q2*Q3: This will make the cost apply to Banners x Length (in feet) x Height (in feet)
Billing Type: Single Unit
Cost Per Billing Unit: The price per square foot of your banners