Hello,
I am using ImageJ to measure the percentage of dyed area on the root of a tooth.
I want to measure the percentage in two ways.

to find total area of dyed area on the root in 1 picture, this means one side of the tooth (there are in total 4 sides, each side is photographed individually).

to find dyed area in each equally1/3 divided part (from total length of the root) please see the attached image for more details.
As I searched for tutorials, the first point may be done by normal measurement, while i am wondering for the second. Please give some advice to divide and select the part of divided and measure the percentage of dyed areas correctly.
Thank you in advance.
Pat