Confused With Closed box enclosure design.

I used Win ISD to simulate a enclosure. It gives me a certain results.
Then I calculated as per given vance Dickason.

I get a big difference. I am not able to understand what is correct. please guide.

Attaching Worksheet as per Mr. Dickason & the WINISD snapshot.
 

Attachments

  • WINISD.png
    WINISD.png
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  • Mathcad - CLosed Box.pdf
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