its to hard to measure
does your port bend ?
does it load near a wall of the box?
does it load near a boundary in the car?
how much power you plan on using ?
do you have a flared port end ? if not port noise will be more audible
there are place on line that have some graphing and math to understand it but in car audio all bets are off its a whole new world when your roll up the windows or open a door or hatch