I purchased one of the cheapie laser levels, and used that to determine my air draft to the closest inch with a tape measure extended to the water line. I also documented the level of fuel and water, and that the measurement was taken in fresh water, it all can change the level. I would not rely on air draft calculated by geometry.
We have transited hundreds of bridges in our cruising experience, and after a while you can pretty accurately judge a bridge height as you approach it. I know that a 24 ft. span will allow me to pass, anything less, the AIS whip antenna will be twanging on the steel overhead. If I get to 23'-7", I will be breaking stuff.
I am amazed that some owners will contact another owner of a sister ship to determine air draft. My reaction to that inquiry is "measure yours". It's not that complicated, and unless you never pass under a bridge, it's information that you'll use time and again.