We manufacture custom cables, most cables have a base formula for the parts necessary to build then we just multiply the cable length  by the price per foot. 

Example

Cable .60 per foot

+

12.00 for labor and materials needed to build

So if customer wants a 27ft cable it's .60 x 27 + 12.00

These products are set up with a -x in our system or as "custom" in the variant fields

I would like the customer to be able to choose the product, then enter the length and get a computed price

Is this possible?