help with port length

[SounDrive Pro Seller]

So if I went 21 I'd be tuned at 25 hz and still be at optimum port ratio?

Actually 20.75 would be the longest you could go, I forgot to account for the thickness of the MDF. But yes you'd be tuned at 35Hz with optimal port area and it would bump you up to 5.06 cubic feet.

Here's the final calculations with a cut sheet from my calculator if you'd like them