flyingsaucer -- use whatever yardstick you feel comfortable with.

Most folks figure their CPM on the 'banked-miles-yield', usually with bonuses & promos added in. Hardcore addicts often include costs for parking, meals & reading-material!
The 'price difference' model is a rather new wrinkle, but if it works for you -- why not ^ It's ALL about the value. Enjoy.

Re: getting near to aircraft. Ever consider popping a few quid for an introductory flying lesson ??