Or you could tape a piece of string from one inner fender (where the hood would sit), across to the other, over the carb. Measure between the string and the carb (air filter base). This is how much room you have for an airfilter and tins.
Your method doesn't take into consideration the contour of the hood or the space between the braces. By your method I would have to cut down my choke tower. Lol.
