I build ducts that direct the air over the cylinders, these are glassed to the cowl so that all the inlet at air lasses over the cylinder heads. On this Byron Glasair the scale exhaust seen on the bottom of firewall had an area of 4 1/4 square inches while the scale inlet openings were 4 square inches, not additional openings are provided. Valley View 40 Twin, head temps under the plug were less than 300F measured with telemetry. With proper baffling you can get by with less than the 3 to 1. Study a full scale, they don't have huge openings, just get the air directed over what needs cooled.