Percent calculator with drawn overlay in C #

I would like to create a program that calculates the percentage of a selected screen size. For example, I want the user to be able to draw a box about anything he wants (possibly using an overlay?), Such as a camera image, and then type in what percentage of the screen size you want to see. So, if you enter 20%, a line or field is drawn vertically that is 20% of the currently drawn field vertically. This line would only have to be movable within the drawn box.

I would like to do this in C #, preferably with a small GUI for the user to enter a percentage. I am not looking for a complete solution to this problem. It is a learning project for me. I just want to give some guidance on where to look for resources and what to do to get a good start.

I have not made a small painting as a small guideline not the best work of art, but should give a hint.

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Many thanks

Christian