i am install 18" x 18" travertine tile with a small grout line. the backerboard is down but isnt exactly level. there are very small elevation variances at the backerboard edges here and there (less than an 1/8 inch). How PERFECTLY level do I need to be. Will the morter between the tile and the backerboard take care of some of the elevation variance or will the slightest ridge in the backerboard create a toe catcher on the finished product? Any suggestions, comments, or ideas are greatly appreciated.