How to finish 1" gap between tub and tile
What you should have done instead is leave a 1/4 inch gap between the backerboard and the vertical flange, and let the tiles hang down past the vertical flange to 1/4 from the horizontal tub deck surface. That keeps everything nicely separated while allowing water to flow down past the flange. 

At this point you need to remove the bottom row of tile. If you can find the same style tile in a rectangular shape that will come down far enough, go for it. Other options would be a completely different (but complementary) tile for the bottom row, or a thin decorative row between the second-last row and the fixed (lower) bottom row.