Assembly rule to add together user parameter totals from numerous parts

I'm trying to figure out a rule, in the assembly, that would go through each part and grab the data in a SQFT user parameter and add the total together for a overall amount. Some parts are used numerous times, so I would need the rule to multiply like parts together for the sum of that individual part and add it to the overall total. I use a 00-00000-00-0 format for the assembly and 00-00000-00-0a, 00-00000-00-0b format for the parts.