if you have some one with a meter cut off and inch at a time to it stops getting louder first find your loudest freq on a tone then test 2 to 3 hrz below and above that after each inch to see if your peek freq changes with the box tuning
you should be fine with the port length you have...tuned to about 31hz at what you have it at now...i wouldnt cut just yet...test first then cut if need be