Unless you see the actual billing the carrier sends to its customer you just have trust them or spot check them now and again by asking to see the actual billing. There is no "set" FSC formula...every carrier is different and many have different FSC rates for different customers.
If your contact spells out a specific FSC rate scale, which is more common, then that's what you get even if the carrier charges more....and most make some money on FSC.
Basically though, all that matters is you get the total rate per mile you need..and that you know your costs so that if fuel jumps say.....10 cents a gallon... you know what that does to your cost per mile and you adjust accordingly. 60 cents a mile plus 40 cents FSC or 90 cents a mile plus 10 cents FSC....still 1.00 a mile and all you need to know is if that's what you need or not.
To answer your question, at L1 anyways, we get every penny of the FSC, but all I care about is the end rate meeting my needs.
The old standard used by many carriers to pay their contractors was
Current fuel price - $1.25 then divide that by the mpg for your vehicle type, typically 15 for a van, 10 for a straight and 6 for a TT. Some would pay a bit more for reefer units or California miles.
So at the current 3.244 for example, a van would get about 13.3 cents per mile.
