Ok, according to winisd a port that is 12.5x4.5 needs to be 18 inches long to tune a 5cu.ft box to 35hz w/ 2 dd9012's. I am going to be using a slot port and the box walls will be part of the port (3 of the box walls will also be walls of the port) Now, JL's site says take 1/2 of the ports width, in this case 4.5 inches so I have 2.25 inches do I subtract this # from 18 to get how long my port must be physically? (15.75inches)

if not, do I have to ADD 2.25 inches to my physical port length? (20.25inches)?

Thanks!