more cfm's for intake than exhaust???

Is it true that you need the intake 2x's more than exhaust? I been reading very many different version's on this. I have 56cubic feet so I'm thinking I really don't need much. Maybe a passive intake with a 4" exhaust and the regular 6" fan tied into my hood and carbon filter. The fan on the light will not be running when lights are off so do I need another source for exhaust or do I just run all time a couple of clip on fans in the tent when light is off? Particulars are 2x4x7 tent, 400 watt air-cooled light so far. Thank you...............................

if you are using a passive intake,you want a larger hole than your exhaust.double the size isn't needed but larger will help. if the intake is too small,it will make the fan work harder & w/ a carbon scrubber,it will already be working hard enough