round port displacement?

[mccleery3 166]

I'm getting some measurments together to do a ported box for 4-SA-8's in a 4cu net ported enclosure. Using 2 of the 4" precision ports I would need each to be 11.90" to achieve my desired tuning of 35hz. I have no clue how to figure the displacement of the ports at their desired length. If someone could give me the formula or better yet just figure it out for me that would be outstanding . I have plenty of time since the subs won't be out till feb.

[KU40 173]

pi * r^2 * length