percentage of area

[michele]

Hi, I have to calculate the area of the blacks stones compared to all other. This is my original image


Imposed on the measurement scale with the command set scale, then I select a square and I click on image-crop:


With a command color threshold I select the stones:


I convert the image to 8bit and I apply the threshold command:


Finally, with the command Analyze Particles:


The percentage of area in my opinion isn’t good, it seems to refer to the entire picture and not just the stones initially selected. How can I do?