Cropping and scaling issue


I have a small concern about next issue.
I have my 30 small images at 40X photomerged together in Photoshop and then cropped to remove excess background.
These images were without scale bars before cropping.
Using AxioVision software, I put a scale bar on one of the small images.
And here comes a point of confusion.
I need to add a scale bar to all photomerged pictures, and I am not sure whether applying it via ImageJ (take small image with the scale bar -> measure the scale bar -> apply this scale to all photomerged pictures) will end up with right representation of the magnification/scale.

Thank you a lot!