Designing vented enclosure

Don't concern yourself with the frequency when you are looking at port noise indication.  The frequency shown in the chart will be whatever you tune the box to and is a completely separate issue from port noise. 

Tune the box to the best frequency for that particular sub and your listening tastes.  Once that is done, you will have decided on a  certain port size - diameter (or equivalent sq inches if it's a slot), and length.  When you input your power and go to the port velocity chart, you will be looking at port noise...which is created entirely because of the port diameter.  If the port noise is too high...(and here again, the frequency doesn't matter, it's just what it is)...go back to your port program and enlarge the diameter.  You will note right away that the port length will be longer with the diameter enlargement, so you will be forced in many cases to reach a compromise between length of port and port air velocity.

I mentioned such compromises in car audio earlier.  Sometimes you just have to reach an agreement with yourself to allow a little port noise so that you end up with a manageably-sized box.

I believe you might be alluding to keeping the port output very low on the frequency spectrum so that it doesn't interfere with SQ.  That is a good way to use a vented enclosure for SQ.  By selecting a box tuning frequency that is very low, and down far on the low end of the response curve, you are achieving more of a SQ response and more similar to a sealed box response.  This allows the driver to have a vented enclosure if that is best for the driver, and keeps you from hearing a peaked range of frequencies.  I do this very often and am currently doing exactly that with vented enclosures for my home main speakers that I'm working on.

The important thing is:  you are doing this by changing the box tuning.  You don't look at the air velocity chart for this, you use the box tab and put in the tuning freq that you want to use.  How low you can go is dependent on the driver's parameters, so pay close attention to the freq response graph (transfer function magnitude) and box/port size requirements while you fiddle with it.

No, and just to be sure, I did a 10 second check on it.  The info I provided is correct.  Do what I just now did as a test:  bring up the air velocity chart.  In your working dialogue box, change the tuning freq to some different random freqs in the sub range.  Notice what the port air velocity does.  You will see that the port air velocity will normally rise with higher frequencies and that every change you make in tuning freq creates a change in port length.  As I said, it depends on the frequency response that you have to begin with.  30 Hz is on a downward slope from 60 Hz, for example, which is to say it is less decibels.  Which means that port air velocity with the box tuned to 60 will be greater than when it is tuned to 30 because the SPL is greater.  It's a matter of power applied at the given frequency.