How to calculate granularity of cells with FIJI?

We would like to measure the granularity of a cell from a 1 plane confocal image (63x, zoom3, so high mag). I mean, a value that can reflect the SSC on a flow cytometer for exemple…
We have cells without any granules in the cytoplasm and some other with high quantity of granules. The marker is specific of the granules. We are not interested to count the granules per se but just want to have a value that reflect the granularity between different conditions.
I didn’t find anything equivalent in FIJI Pluggins but sometimes it can be under a specific name that I will not associate to what I’m looking for.
Any idea ?

maybe the Standard Deviation might reflect the SSC parameter.
Check the SD difference between cells with and without granules?