Hello
i am trying to place a button on the image in a specific position of Close button in the image as you can see
in the Scene tab it is ok and perfect
but when open it in play mode for portrait mobile
it is not the same like in the scene tab
so how to Adjust it perfect ? 
place the anchors on each corner of your button, it will size prefectly no matter which size your screen make:
you can see on this screnshot that I Set the anchors to each corner of my button. It now keeps its relative size on every screen.
Let us know if it is not what you asked for of if it works for you 