Finally got them all. It was pretty hard.
At first I couldn't complete the first level, but then I noticed that if I exited the level and zoomed in on the navigator tab the circles got bigger, and after increasing it several times I finally managed to complete the first level. Afterwards it was measuring shots and repeating them again and again until I got them all.
Nice game, it would be nice to have an additional scene or image at the end once you get them all with gold.