Your jacket should be half as long as the distance from the top of the collar to the ground i.e. should split your body vertically without either making your legs look stumpy and torso overly long, or your torso small and legs too long.

Easy way to do this if you are on your own is to just take a jacket that you have, ideally one that fits but not necessarily, put it on, put on dress shoes (to account for the added height of the heel), put one end of your tape measure just at the heal and the other end to the bottom hem of the jacket. Then take the jacket off and measure the length of the jacket from the collar to the hem in the back. Add the two numbers together and divide by 2. This is the length of your ideal jacket.

This is superior than just getting your girlfriend to measure you because 1) You don't need a girlfriend and 2) it takes into account where the jacket collar sits naturally on the back of your neck and the fact that the jacket isn't going straight down but must follow the contour of your back.