Just for a quick reference when others may search this same topic.
The safest way to figure it, is by using the shorter end. It is not "exact" by any means but you will be far happier if you tuned a tad bit to low instead of to high.
If you want to try and learn a better way you can do some reading on enclosure design, port area, and such.....once you get a good grasp on that you can start looking at end correction and other variables that will play a factor. Once you start getting all that down, you will be able to figure out just about any port area and tuning design you may need. It is the long way around, but a far better pay off if you want to learn along the way.