Why is it that you want to measure with Max at 255? Usually 255 is to be avoided as in 8-bit images, you can’t tell the difference between 255 and 255 if the latter actually would be, say, 400. So 255 means your detector (camera?) is saturated. So my guess is that (large?) parts of your image are overexposed. Can you re-record images, or are they a given?
Especially when you want to calculate (test-control) values, you don’t want to scale your signal, as you seem to request. When your imaging device allows, you should use 16-bit, or at least more than 8-bit, if you want a higher dynamic range. That then allows for direct calculation of the test-control, if the settings (exposure time, intensity of illumination, incubation settings etc.etc.) are kept constant between recordings.
Best way to go is to have calibration objects, where you calibrate your fluorescence against known values. That may be in arbitrary units, or known concentration of the fluorescent dye, or known amounts of the stuff you are labeling. I take for granted that you have a specific channel for each dye.
As an example of how you could (mis)treat your images and do get 255, run this little macro below It shows the results you desire, as well as what now is considered the “truth” (an image with overexposed blue channel, and a sparse histogram).
run("Fluorescent Cells (400K)");
run("Enhance Contrast...", "saturated=0.3 equalize");