I think this is a little too involved of a task for the Power Store, the problem is the ranges bit. To make it work in Power Store you would have to configure each possible combination and set a price for it on a per item basis. This is not something that I would recommend.
Another way to do this may be to have a table in your db that will hold the lengths and have columns for the widths, 12, 24 etc.
You could then in your code select the appropriate column values based on the entries of the user. There are no features or interfaces of eCart that would help you craft this setup, it will be completely custom.
