If you take the weight of a seat and compare that to the weight of the rest of the plane it is most likely negilible. It might be as simple as "how much money do we need for YQ?" and then creating a formula that comes up with the desired result. Or it might be the square footage taken up by a CW seat vs a WT+ seat. Or it might be the square root of WW's birthday multiplied with the expected date of the next strike/volacano eruption/fog/snow/... incident.