The easiest way to measure is level your trailer with a level. (In your case ask the dealer or owner.) Once it's level, measure from the ground to the inside of the receiver - the space the ball fits into. That measurement is how high you want your ball to be, and you can figure out the rise or drop (if any) that is necessary. I would use a slightly lower setting if I couldn't get it exact.
Also, I had a 1207 a couple of years ago (... a good trailer). It is NOT a problem tow. If you picked it up at the dealer and it was a little high or low, you will get it home no problem. Good Luck, and enjoy your 1207.