Honestly it's not that bad. If you know how to do basic math is to add and multiply. So 10 4inch louvers let's say equals about 1 square for of ventilation.(math is a little tricky on those) As for adjustable floor vents treat it as a standard vent length times width as a gable vent. But how much ventilation you add comes down to the type of bird(heavier breed need more ventilation),type of bedding, poop boards, frequency of cleaning,cross ventilation etc. The link above is a very good reference to even the type of climate and how to get your numbers of how much real ventilation you have per bird for the coop.
My point being, however, that if you're talking about a 4"x10" floor vent -- which is not actually 40 square inches because of the louvers blocking part of it -- when you need 144 square inches or more per bird you are thinking on the wrong scale.