ok thanks i understand better, so does that formula work for any frequency? ex: 1/55=0.018ms and by one cycle do you mean like one single note or what do you mean by that? So basically as long as your group delay is less than the 1st and 2nd cycle of that frequency there shouldnt be any audible difference? my group delay at 40hz is 9.5ms, so thats good right? Im using bassbox pro, is the goal basically to get the amplitude response as smooth a slope and as close to 0dB as possible for best quality? my f3 is 28hz and the slope goes no more than 0.45dB above flat line up to 100hz is this ideal?
That formula is not really a formula, but more a back-of-the-napkin rule of thumb kind kind of thing that usually works. It's better than nothing, but not perfectly reliable. So yes you can use it for any frequency, just don't rely 100% on it.
Yes, I mean as long as the group delay for any given frequency is smaller than the time it takes for one cycle of that particular frequency to complete (if you were playing a sine wave at that frequency) then the group delay should not be audible. In theory. Like I mentioned, there have not been very many good listening tests to determine this and people argue about it all the time.
Yes, 9.5ms at 40hz is pretty good.
Using BassBox Pro is pretty straightforward. The goal is to select a woofer and design a box that meets your low frequency requirements. Most people want a flat response down to 20hz, low group delay, minimal hump above 0dB, and a design that the Xmax of the woofer is not exceeded with the power that is planned to be used (ported boxes usually need a sub-sonic filter to help with this). This is a balancing act between enclosure volume, port diameter, and port length.
Some people don't realize at first that subwoofers in a car gain a tremendous boost in the lowest frequencies and a "flat" response all the way to 20hz is not needed. Having an F3 (-3dB point) down to 30hz or even 35hz is sometimes acceptable depending on the kind of music you listen to.
Last, the most important part of the article you linked is:
This higher tuning is what gives ported boxes a bad name as far as their sound quality is concerned, but when ported boxes are designed correctly, this issue doesn’t exist.
There are TONS of fantastic speakers out there (some of the best actually) with a ported design that also sound "tight" and don't have any sound quality issues. Please don't fall into the trap of believing that a sealed box is the only one that can sound "good".