When it aks for the box volume, then the desired tuning frequency...are you supposed to enter the size that you want the box to be AFTER vent displacement?......
..And then when it tells you the length your slot port needs to be, do you then figure up how much space the vent will take up, then add that to the size box you wanted b4 vent displacement to = the total internal volume your box needs to be?
Anything that takes up space within the enclosure must be taken into account ... Ports, braces, drivers, etc.
Also, I don't trust the loudspeakers101 calculator. Use WinISD Pro, JL Audio's formula, or diysubwoofers.org's formula ...