I need to keep my min threshold value at 0 but be able to manually set the max for each pic

Any help would be greatly appreciated.
I am doing densitomitry analysis on bright field jpeg images. I have learned how to record a macro/ convert my images to 32 bit and all that, but I still need to manually adjust my threshold max. I am using the default, red settings. When I draw my polygon shape, push “T” threshold–> “M” measure I get my first reading, with the minimum at zero like I set it. However, when I take my next polygon of the same or different image, the minimum value starts to change. How can I keep this value at 0 throughout my whole batch of pictures?

Hi, Jpegs are not a good idea

Also there is no need to conver to 32 bits because the threshold methods are for 8bits (some also for 16 bits), and 32 bit images will get re-binned into 8 it space (so you do not get any additional benefit.
Oh, and for setting the lower part of the threshold to 0, it would be easiest to use a script. You can try with the Macro Recorder and use the Set button in the threshold applet to set the values. That will show you in the recorded macro how to set the values.