Not that it's right or wrong, but I don't calculate the mileage. I just charge what it's worth for me to to the job and take the distance into consideration. Probably the way Scott does it is best though.
Generally if someone likes you enough to book you a long distance away, the money isn't that big a problem.
Ten years ago I once booked a wedding reception in Dallas (150 miles) for $1000.00 plus a motel room.
When the guy paid me, he asked if I needed more money for the travel expense.
Somehow I told him no!!
DonM