Calculate area with variation of introduced measurements

Hello,
I have a carpet product that I sell per square meters.

The product is in a roll of 2 meters wide, so if the customer enters a measure of 1.1 meters wide for example, I want the area to be calculated as if he had entered 2 meters (to charge him for the surplus of the roll).

Where should I apply this, in the “width” module, in the NOV or in the formula?

I have additional options for this product, the price per square meter is also calculated but I would like the formula not to affect it (that the area calculation is the exact one that the customer enters).

Thank you very much.

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