It can be that way, yes. But unfortunately it’s not. The preview you have in the designer only works in pixels while the way you are proposing is a proportional or relative positioning.
I’ve done a lot of apps that places stuff at Screen1 initialization and it doesn’t take a lot of time really. Just make those calculations you posted in your last comment to be able to place the balls in the right spot. Just remember to use the canvas width and height values instead of the Screen1 width and height. In fact you can create a little procedure that will accept a percent for width and another for height and the sprite or ball you want to position, and it will reduce the number of blocks needed a lot.
The example will place the ball exactly in the middle of the canvas. 50% width and 50% height of the canvas. You can give any number from 0 to 100 on both and it will work too.