Port length calc discrepency

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Hello All

I've finished my first home built subwoofer using a design I (and others here) worked on in WinISD. Anyway, I've stumbled upon a dilemma in calculating the port length for my box. The port I'm using came from parts express and can be found here:
3" Precision Port Kit

-WinISD, says that for my driver with a tuning frequency of 30hz in my 59L box, and dual flared port ends, I should have a length of 7.56".

-Using the spec sheet that came with the port to calculate the length I'm getting 7.966"; which it then says to add an inch to (I assume due to the flare).

Playing around with differences in port length using WinISD shows that changing the length of the port doesn't effect the tuning frequency much, but it really does a number on the frequency response of the system. Thus I'm trying to be as accurate as possible.

Now, which should I go by? The difference here is quite a lot, 7.56" vs. 8.966".

Please help:confused:
 
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