Microsoft Excel - how to calculate / draw FWHM?

[Jimbo Mahoney]

I work with microscopes and sometimes need to draw / calculate the full width half max (FWHM) of the intensity of a sub-resolution bead image.

How can I do this in excel if I have the graph and data?

I can easily calculate the half max, but then how do I get the width of the graph at that point without just eyeballing it?

Many thanks