Ubercart

This is a job for CCK. You should start by making a Flooring product class to make a new node type. This will keep your different product types separated if you ever start selling anything else. In the Content types administration page under Content management you can add a "Surface Area" field to the Flooring node type. There are many different options, but it sounds like you want a Number field with a text box widget.

CCK is a very complicated set of modules, so continue to explore all the options. When you create a Flooring node, you will see the new field you created, ready for input.

As Lyle suggested, I think a CCK Decimal field with a text widget would do the trick for storing the Surface Area of the product. Then if you add a product attribute with no options it will display a text field for the customer to enter the surface area required.

At that point, you could modify that ajax module to calculate a quantity needed but there really isn't any need for ajax to be used at all. A little bit of javascript should do the trick for you if all you need to do is the calculation that was mentioned above.