Slightly off topic here, but it seems like most of the cells you are trying to detect are grouped together, and have a similar “feel” to their texture. It might be worth looking into a pixel classifier to identify those regions, and then you can use the pixel classifier to classify the cells (one of the options, you can also create objects, but I think cell classification might help you more here).
Not certain how well this will work, but it might be something to consider if your current methods are not giving you sufficient accuracy.
I am not sure using the pixel classifier this way is covered in the link, but the link has most of the information you will need about setting up the pixel classifier. The pixel classifier can be very stressful on a computer, though, so if you do not have a powerful computer, you may run into problems (I just had issues on a PC with 255GB of RAM).
I’ll leave the specifics of the measurements to Pete, but I imagine regardless of how it is calculated, the mean value would be incredibly diluted by the entire rest of the cell. The membrane (the very edge pixels) is probably less than 5% of the total area… so it won’t have much contribution to the total.