I am building a 118L vented box tuned to 20.5 fb. using pe's 250w amp and shiva mk IV driver. unibox says 17" port and winisd pro says 16.5". that is basically the same. I bought the 4" flared port kit. Is the length the overall dimension or the center section, not including flares? the assy is roughly 18" overall. the straight section is approx 16". so would I have to add or subtact and 1"? actually i am going to do nothing just curious for future reference.