I've done the 60/40 where I've been responsible for fuel. Always needed the advance after departing shipper to cover fuel costs. Still profitable, but if you had the choice would you prefer taking 40 w/ fuel card or 60 & no fuel? Which is typically more profitable?
Yeah, that last part about restricted movement would probably irritate the heck out of me as an operator of my own business, really enjoyed the freedom to choose when and where I wanted to relocate.
That's also a great way to look at it (the 20% covering fuel). It probably should have been obvious to look at it that way. For the most part yes, it was only when the carrier would strand me w/o a load for days on end that the fuel costs of just sitting would make money tight.