Air flow reduction??

Air flow is gowing to depend on a number of factors including the air velocity, material of screen, amount of contraction/expansion of air flow, and size of screen openings. It will not be linear. As such, you can only get measures of reductions in air flow for specific materials/screens at specific air velocities. There will not be any fixed reduction percentage.

Get a pitot tube (they are cheap and can measure very low air velocities), a manometer (to use with your pitot tube), windows (screen in and out), calipers (to measure screen wire size and spacing) and maybe some cardboard (to play with area) and go have some fun on a breezy day. This will get you going in the right direction.

Based on the velocities you can calculate air flow volume over time. It would also be affected by how "tight" the house is. Try it with other windows open and closed. Measure windows on different sides of the house.