Fireplace Mantel Video

How do you get the mantel flush to the wall so there is no gap between the top and the wall?

I am building and handing a fireplace mantel shelf to a rock face. I will be using leg and leg screws. My plan is to drill through the mortar instead of the stones just in case one would want to move the mantel shelf. This will allow you to hide the holes by martoring over them. I will mount the shelf directly to the rock face without using a backer board. The shelf back board is 2x6. What size leg screws would I need? My mantel shelf true size will be 5.5" (H), 9.5" (D) and 68" (W). The back board is the only board that is 2x6 all other board will be 1x6 and 1x10. I am also try to determine how deep into the martor to drill the leg holes and if drilling through the martor will work for a shelf that should be between 25 - 35 lb. Thanks...