TOPIC: Tiles display issues

Hello,
I am trying to use Tiles extension to present my Joomla website home page with tiles.
The only issue I have is that the tiles do not fit exactly centered in the mainbody position of Protostar template. For instance if I have 3x2 tiles, I have to play with right or left arrows to see everything. I thorougly took in consideration the width in px of the pictures vs mainbody total px available without success.
I am not sure to explain the issue properly and therefore I have added screen copies
Thanks for your assistance