I can't recall where I heard this and I'm not sure it holds true for all situations....

If you run a string from the top of the windshield to the leading edge of the hood/header panel, the opening (on pro-style scoops) should be outside the string line.

Of course, not all carbs sit at the same height inside the scoop, so proper internal ducting would be required.

Maybe a chassis shop like RJ, or Bickle can share some of their aero info.