headlight measurement

[kiwi]

I'm trying to source a Fly screen for the bros 400 I own, and my searches I find this statement a lot

will fit most standard 7 inch headlights

Is the measurement taken across the headlight lens or between the headlight shell mount points/dia of the headlight shell because there is 3/4 inch difference ie the lens measures at 7 inches and the headlight shell measures at 7 3/4 inches.

I ve emailed the manufactures of the the ones I'm interested in and they don't seem to be interested in replying