This is a classic example where you sell a product by square meters with a free length, and let the customer choose the width based in standard rolls widths.

You could add limits on length values or use a fixed list of valid lengths.

// Get customer values
$cuWidth = {17V};  // There is implicit conversion to meters here because values have "centimeters" as unit
$cuLength = ({18V}/100); // here we must convert to meters manually as expected in "centimeters"

// Calculate Square meters
$SquareMeters = $cuLength * $cuWidth;

// Set final price
$FinalPrice = $SquareMeters * $Price;

You can visit this demo in our Demo Store