Graphic problems with pentagram

Graphic problems with pentagram
Good morning to all, I’m creating an app for my nephew who train the musical ear. In an image with pennagistic pentagram and keyboard (Photo1 - 1024x488),

a note Random (Sprite or Ball) appears of the 48 notes available on the Pentagram, and the player must say what note is. Simple.

In the project with Screen2 Landscape Scorrable, I added a canvas with height and width properties “Fill Parent” with the image property of the pentagram and a ball.

My problem is specifically indicate the position of the ball on the pentagram (two pixels of vertical difference can change the position of the Ball on the row or in space and one and is not a f). Already the screen in the editor is all staggered on my monitor (photo2)

, instead of my companion (I have a smartphone) the image appears proportionate (photo3).

Since I can’t set up a table arrangement on the Canvas, I thought I was approaching to the problem by calculating the precise X and Y coordinates for the 48 positions of the ball on the pentagram of my smartphone, but I would not want to change the device all these coordinates no longer served for a Change of proportions of the canvas image.

I don’t have a second devide (a tablet) to try. What do you think about it? Am I having a right approach to my program interface? Advice?
Thank you

No one has any idea how to help me?