2) Make a large circle, I have used the rulers to make it 30x30 units.
3)Copy and paste the image of your prom, it should now be in a separate layer.
4) Select the circle layer and click Edit>Free transform.
5)Click the link icon in between the W 100% and H100% icons at the top. This allows the rescaling of an object to be the same in both the x and y axis.
6) Rescale the circle until the curvature of both the circle and limb on your image match up (this will take a bit of trail and error and is a bit easier if you alter the transparancy of the prom image).Rescaling is done by clicking a corner of the box that now surrounds the circle. Once they match up, the circle now represents the relative size of the Sun to your image.
7)The Sun is about 108 times larger than the Earth. So we need to rescale the circle so that it is 108 times smaller. Select the circle and click edit>free transform. In the tool bar at the top again click the link icon and enter in one of the boxes either side 0.93%. This will rescale the circle to make it the relative size of the Earth.
8) Move the Earth circle over the image of your prom. Then use the crop tool to remove the surrounding area around your image.
You should now have a circle which can be copy and pasted for any other image as long as you have used the same imaging setup e.g. cameras and barlows
Here are a few images that may help.