I am in the middle of a bass guitar cabinet project. I've gotten most all of the components and am ready to start but I have one small prob. What size to make the port in the cabinet. I talked to the guitar and combo salesman at my local music store and he said there is some sort of formula or equation you use to get it but he couldn't remember what it was and he refered me to the internet. Not much luck there either. So if anyone would happen to know what this formula is or have any info that would be usefull to me it would be greatly appreciated.