If you insert a smaller image into the 800 x 800 canvas, Paint 3D will center it by default

However, it will be centered as a rectangle - meaning, if you draw a circle around it, the CoA will not look centered

You need to think for the CoA to be rather a triangle, and then this would be centered if the 3 edges are at the same distance from the circle

So, as a rule of thumb, you need to lower the CoA a bit, for it to look centered

With some experience you can do this manually

But there are tools to calculate the exact position, and also templates for GIMP

This very much depends on the material you have - Guess & verify

The 350 size CoA has transparant background, so it can overlap with the map (allowing it to be of that size)

Make sure the CoA does not overlap with the highlighted part of the map

When using Paint 3D, you can quickly draw a circle that fits into the canvas, to verify if the image looks centered

Press esc to make the circle go away again