1st you need a termlab.....then take a sub, like a 10" sub in a 1cuft sealed enclosure and run a sweep....should give you 2 peaks...for example when testing mine I had a 40hz and 53 hz peak. from testing with ported enclosures in my vehicle i tune them about 4-5hz below resonant frequency, so for my 40hz peak i have an enclosure tuned to 35-36hz for daily and another tuned around 48hz for comp ...all vehicles are different tho..i've seen some have 10+hz jump from tuning frequency.
but yeah lots and lots of testing to know your vehicle and how it reacts to different variables...i.e. port length, port area.....box and port positioning...box size...power..etc...test, test, test //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif