Tiling kitchen spashback with gap between worktop and wall
I like option three.  

Put down painters tape first then slide the ruler to the wall and tape it down securely along the edge away from the wall. This gives you straight edge to work on.( assuming your counter is straight ) 

Once your tile is set ( 24 hours ) you can pull the tape, gently slide out the ruler and remove the other tape. 

If you still have a small gap the  caulk between the countertop and the tile should cover it.