I go by the time and distance it takes for me to get to my outcall. In Central London where the only cost is the price of an off-peak travel card, and it only takes an hour or so to get to the booking, then I don't charge anything. But if I have to travel anywhere for more than 90mins, then I balance it out by saying that my booking time has to be the same amount of time. I.e., a 90min booking for having to travel for 90mins one way; a 2hr booking if I had to travel for 2hrs one way. And if my fare cost me more than a tenner, I'd inform them at the time of booking that travelling costs would apply. For airport visits I charge a tenner for petrol. Customers never had a problem with that. As I say, you tell them at the time of booking, so they don't have any unpleasant suprises and they don't think you're trying to scam them out of more money.
When I first started and I didn't know any better I used to travel up to two hours for an hour's booking, and it was just silly. It would take a whole morning or afternoon, just for an hour, not to mention the getting ready beforehand. So now, for however long I have to travel one way, that's how long the minimum booking is.
It works for me.