Hi there, I need help on how to calculate the percentage of missing objects using Photoshop.

You could do this approximately by using the Pen tool to draw the estimated whole as a path, and turn it into a selection. Look in the Histogram panel, under Selected Layer, and you can see the number of pixels selected; note this down.

Clear the selection, and select just the fragment with the Object Selection tool. Find the number of pixels in this as above, and compare the two values.

Assuming just a basic wheel shape, a quick test shows your fragment would be about 54% of the whole.

I wanted to go down to "create the solution  yourself route" and provided information in that direction but what @Kevin Stohlmeyer said above is much better and faster.

I didn't know about area, perimeter & circularity myself. Thanks Kevin!!!

Export the area to a txt file from the measurement log after taking a measurement, in a jsx load the exported txt file and compare the area to the total image size. Hey presto.

Thanks for the excellent question! I learned something new because of it! ❤️

Have you considered the Count tool in Photoshop?

https://helpx.adobe.com/photoshop/using/counting-objects-image.html

You could theoretically use the count via selection with the units set to pixels to count the selected area.

Then inverse the selection to get the remaining part of the image. Calculate and there is your percentage.