This image is a condensed version of the linked thread that 3catcircus posted:
I created my own drawing tool on the map which looks like this:
I have an expanded version of this to allow me to contruct courses to set to, to move in certain relative motions around the target. But this is complicated enough for now.
I draw this anywhere on the map. As your position is not actually plotted in that drawing. It only needs a line (light blue) for the direction from you to the target, the true bearing to target. As that is the direction you want your surplus speed to be pointed to close the distance.
You can get the true bearing from adding the periscope bearing to your course (and reduce it to within 0-360 degrees.
Or you can use map tools on the map with bearing-compasses on the mouse cursor to measure it (like this one I made:
https://www.subsim.com/radioroom/sho...postcount=1308)
If you draw a line from your current position to the target position then you can shift the target-speed circle radius arrow ONTO you current location. And draw the rest of it based on the origin of the circles.
The arrow/radius of the target circle (red) is a scaled version of the target speed. And it's direction is the target course.
The arrow direction of your speed circle (black) isn't important. As the intent is to figure that out. The
length of the radius/arrow is important, it is the speed you move at during the intercept. And needs to be in the same scale as the target speed circle.
The intercept course is the direction of the green line.
If you move, then you need to re-draw the whole thing.