What I find is that nearly all of our customers have their own fuel scale that they use. When it's an American customer, it's generally a rate per mile but different shippers may use different peg levels as the baseline for their fuel ... as long as it scales normally, it doesn't matter.
Canadian customers tend to use FCA fuel. Most discount it ... so they'll pay anywhere between 60 and 75% of the FCA posted recommendation. Personally, I think % based FSC is BS, because it assumes that the rate per mile you are charging and the percentage of fuel expense in relation to total expenses is the same. And we know that this isn't true.