bob
It's better to have two hands than one glove.
Think you still sell by the square foot which is out dated.
You're correct if you're referring to a simple minded square foot model as in area * rate.
But, if you're not profoundly innumerate, there is nothing wrong with integrating a complete pricing model around area of the product. Done properly it yields sound and consistent pricing. Far more consistent than the legion of products currently available.
As long as the model is reasonably correct you can express the results by the square foot, by the pound, or by the cycles of the moon. It really doesn't matter how you express the results, it's how you arrived at those results.