speaker vent with focal kevlars

hey, just got done building my first set of monitors using focal 6K4311 (http://zalytron.com/Specs/6K4311.pdf) and a tpcbjm tweeter (customized version of the tc90, that was used for jm labs )

well anyway. i noticed that it is recommended that with the focal midrange to use a vented enclosure, can some one explain to me how to calculate this, or what to do.

dimensions for cabinet
7 3/4 in width
11 1/8 in length
18 in height

with 1 1/2 inch depth of wood.

any help/suggestions would be appreciated.