Don't you use the shaker bracing on the bottom of the hood as the guide, less whatever the crimping is going to take up?

I'd hate to get a set of measurements for a shaker hole. You'd need a degree in engineering to use them if they were accurate.

From where would you measure? The left and right front corners to the apex of the opening for the starting point and then what? The arc tangent of what to where towards the back for how far?