ProductCart Upgrade and Modifications

I am looking to upgrade a custom ProductCart from v4.7 to v5+. My current ProductCart has the apparel add-on and has been customized to allow a customer to order customer rugs based on square footage. The customer selects the Width and Length in feet and inches and a price is generated based on the square footage. The length and width and prices are controlled by text in the Product Notes under the individual product. Here is a link to a page in the store front that shows a product that allows a custom price: [login to view URL] Here is the text in the Product Notes that controls this function: {W12-168||L12-168||SF196=$5.00|SF196=$6.50} The W12-168 sets the dropdown menu for Width. The L12-168 sets the dropdown menu for Length. The minimum is 1’(12”) and the 14’(168”). The first SF is price per square foot up to 196. The second SF196 sets the price per square foot if a person orders a rug that is more than 4 feet wider than the length which we consider a hall runner. It looks like the customization touches about 25 files in 4.7. I am open to change how a customization to figure custom prices by the square foot is handled especially if it will make future upgrade easier. Would also like to explore the possibility of a responsive site design.