Air exhaust/air intake ratio

Anybody know what this should be (using CFM units)?

1/1?

or say 1.5/1 if the exhaust has to push through a carbon filter.

Personally I don't worry about intake very much. It's good to keep the garden at a negative pressure, especially if using a carbon filter. If you don't have negative pressure then smell can leak out instead of going through the filter. Air will always find a way into the grow area, through doors, zippers, cracks etc. Concentrate on the exhaust and the intake will take care of itself. If you really need an intake port then make it very small in comparison to the exhaust. I run an 80 CFM fan exhaust on a 3'X5'x8' closet. No intake. I have black and white with a zipper in as the door and it is vented through it. With the fan on, there is no movement of the black and white moving from the negative pressure. The zipper allows enough air flow for me. Exhaust is what you want to focus on.