Ah, assumptions :-(, the expression is only 0 at x=0, looks like a capital M ...
For ## \delta = 1 ##, the ##max^2## is ## 2 \times 0.664 ##, but I would need to know this in terms of ## \delta ## to use FWHM.
slightly off-topic to help me get a feel for these things, what would be an approx...