Arranging Tiles on the main launch according to Usage pattern would simplify navigation. 

This is the usual 80-20 rule. 

Please let me know what MRU means. If you want to custom the number of app tiles row, you can check the following registry.

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell\Grid

For more information, please refer to this article.

http://www.eightforums.com/tutorials/2690-start-screen-metro-apps-change-number-rows-windows-8-a.html