Port Measurements Dual Sub Single Port

[FuriousG]

Trying to design an enclosure for 2 X18s and am unsure how to calculate the port length with dual sub shared chamber.

I am aiming for 13ft net, 50.5Wx31.5Hx24.5D, port 28.5x7 and should be roughly 25" long. 2 common walls (center port) subs and port forward.

Where I am stuck is that torres shows physical port length at 16.5". If I measure out 25" effective port length per side I end up with 19.5" physical, which is 3.5" from back of enclosure and then end correction messes with me.

Could someone give me some insight?

[FuriousG]

[Joe X]

Port length is measured as follows:

L1 + L2 = what you entered for port length in Torres:

Never mind about end correction and else, the calculator takes care of that.