Help: Port Length Question

[x6369x]

I need some help calculating port length ...

I have an Atomic APSPL12 that I'm putting in a 2.0 ft3 box per Atomic's design:

2.0ft3 4” port x 12” long (36Hz) F3=39Hz 600w

I'm using a 4" Aeroport and the box will only have an internal depth of 8.5", so I will need to use a 90 degree elbow.

IIRC ... for a 90 degree bend, you have to add 1/2 the port width, so the overall port length will be 14".

Is that correct?