Advise needed on port tuning

Hi guys. New to the forum. I’m just building my first decent install. I have built a box but I’m stuck on the port length. The box outer dimensions are 36wx13hx13.5d outside using 18mm 3/4” mdf. The box internal volume is 2.7cu without the port. Port opening is 3”x 308mm but I’m stuck on how long I need to port to be. My sub is a bassface spl12.2 and has a recommended 2.5cu with a port tuned to 35hz. Any help would be greatly appreciated