That port calculator is a little off squeak...but it will still give you a general idea stish if you use it.
For example, a 3.5 cube box tuned to 35Hz with 40.5 square inches of port area should have a port lenth of 20.19, but the calculator says 18.7 if you enter the port dimensions one way and 13.45 if you switch the width and height (shouldn't happen, as it's just a function of area). So...I reccomend looking up some equations using the search button on here or get some help from someone. PM me and I'll help you out if you need it.