That's assuming the other costs (refining energy costs, transport, the company's gross margin) are uncorrelated to the price of crude oil, which seems unlikely
A) Just calculating the percentage doesn't assume that.
B) They shouldn't correlate by a particularly large amount in a competitive environment. For an approximation as rough as "half" and assuming no other changes it's not a big deal.
B) They shouldn't correlate by a particularly large amount in a competitive environment. For an approximation as rough as "half" and assuming no other changes it's not a big deal.