Hi,

The general rule of thumb is 1/3 to 2/3. That is the opening to let the air out should be as close to twice the size of the opening to let the air in as possible. The object of the exercise is to get air flowing around the motor. Large openings often create a situation where the air doesn't flow if these proportions arn't followed.

Other compounding factors are, is the head exposed ?(in your case this is good), how much of the muffler is also enclosed in the cowl and the shape and location of the cooling entry and exit vents, oil ratio and type, throttle setting, etc, etc.

From your photo I would suggest that the opening at the front to allow the air in needs to be larger and located where it will scoop the maximum amount of air. The proposed exits look fine providing the proportions with the inlet are kept. The exposed head really helps and will give a margin for error.

Good Luck,

Colin