How do you measure the port length and area of a dual sub box with a center slot port?

[Milky]

Im planning on building a box for 2 Sundown SA V.2 12" and have dimensions of 31"W x 16.5"H x any depth. I planned on using the recommend area of 1.75'^3 for each sub, giving me 3.5'^3 total net volume. I am willing to adjust the depth to reach said 3.5'^3 net volume. Im using 3/4 MDF board.

Now, using www.subbox.pro, I have linked my box that I want, tuned to 35Hz, a big of a port that I could get, say I make the center port, I do need space off of each wall equaled to the ports width. It tells me that my port length is 28.24". How would I go about that, if the port is shaped like a " T "? This image I made, kinda shows what I think it would be. I think It would be measure like the red line. But ive also though of the green too. If it is green, would I have to halve the length of the horizontal port piece to make sure that the total would still be 28.24"?

 

Also, I did make the horizontal ports halve the port width (2") off the back wall, to keep the same port area (4" x 15"), since im going both ways? Is that what im supposed to do?

 

Thanks.

[Joe X]

The total length of the port your calculator gives should be equal to L1  + L2 in the figure:

 

 

You draw all this on the bottom panel, from there you can figure out the length of the port parts.

 

Just bear in mind that since there are two sets of port parts for the shared port,  left and right, the net internal volume is not the same that would be in a box with a port to the side that only has one set of parts.