In the documentation in the chapiter " How to purchase in different unit of measures than sales?", i find  : "For the Square Meter, we will specify that 1 Roll = 10 square meters of curtain. It will thus be necessary to specify that as type, the square meter is bigger than the reference unit. The Bigger Ratio is 10 as one Roll = 10 square meters." But how do when for a second roll we have 12 square meters ? And for another, 17 square meters ?