Paul Carmody's description of the Hitmaker kit says the length of the port should be 6", but it is unclear how to measure the 6". Should I measure the 6" from the outside (back) of the cabinet or from the inside. With a 3/4" thick MDF cabinet the wrong choice would mean that the port is 5 1/4" or 6 3/4". I don't know if that makes a difference, but I rather get it as Paul recommends. Thanks.
Whether a port length is derived from WinISD, a Port Length Calculator, or from a loudspeaker designer's specifications, it is the full length of the port itself, no matter the thickness of the cabinet material. However, bear in mind that a flared port would need to be somewhat longer than a non-flared port, depending on the radius of the flared portion of each flared end. This is because, all else being equal, the greater the port area (or diameter), the longer the port must be to achieve the same Fb (tuned frequency); as the flared potion of the port has a greater area than the cylindrical portion of a flared port, and this greater port area must be offset by increased port length, or the resultant Fb will be commensurately higher. In other words, it is the volume of air within a port that is important (and the greater the volume, the longer the port, for the same Fb), and a flared port has a somewhat greater volume than a non-flared port of the same length, which would result in a somewhat higher Fb than a non-flared port. However, as Fb is determined by port area, port length, AND cabinet volume, a longer/shorter port will displace greater/lesser cabinet volume respectively (and the greater/lesser the cabinet volume, the lower/higher the Fb respectively). Lastly, with high port velocity comes port noise (known as "chuffing"), and a flared port will produce less noise than a non-flared port, at a given port velocity. This is not to say that non-flared ports are always noisy; but rather, they will become noisy at lower port velocities than flared ports - with single-flared ports being somewhere in the middle, between double-flared (a flare at both ends) and non-flared ports.