Fuel Surcharge

Don’t want anybody to give away their trade secrets here but I’m curious how some of y’all arrive at a specific fuel surcharge rate. For instance, I’ve seen one individual I’ve pulled for charge a 45 cent per loaded mile fuel surcharge. I watched that rate fluctuate anywhere from 70 cents to 30 cents depending on fuel price. But I haven’t been able to figure out how they are arriving at that specific rate and also what formula they use to adjust it in relation to fuel prices. I need to bid a job and it’s gonna have a built in fuel surcharge but I want it to be bulletproof and have some actual mathematics behind it to where it simply follows the price at the pump regardless of how high or low it is instead of just being a random shot in the dark. Or are folks just shooting a figure that seems workable to them and it doesn’t really have a formula to support it?