lining things up with the good old eyeball has always worked for me.
With the air horn on the bench upside down and the float assembly in place, the air horn casting AND the gasket give you a real good reference to what perfect alignment would be.
I agree with Stuart.
The "side to side" alignment is checked visually. Just make sure it is centered as close as possible. Bend each side to center as necessary.
If you don't trust the eyeball, use any measuring tool.
