Generating Ratiometric Images from IVIS

Hi! I have images obtained using the IVIS–one is the 500nm signal (green) and one is the 580nm signal (red). I want to obtain the ratio of 500/580 using the image calculator of ImageJ, but am not sure how to go about this by making sure my 500nm and 580nm channels are edited equally (specifically the max min values, brightness/contrast settings etc) and are comparable, before I can do the division. IVIS images of both wavelengths were taken at equal exposure times and settings. Signals I obtained from IVIS are quite weak. Raw grayscale images are attached. Help would be appreciated, thanks!

Day 12 pH 2 - 500.TIF (112.8 KB) Day 12 pH 2 - 580.TIF (112.8 KB)