Flooring Tile - Question about tiling

I am going to be tiling over the existing tile on my bathroom floow. The tile is ceramic over ceramic. The tile on there now is in perfect condition. The only question is, there is about a 5/8 gap or so where the tile and concrete substrate stop and the side of the new fiberglass tub start. The substrate fo the tiles is a 11/8" concrete slab. My question is: Can I just fill the gap w/ self leveling concrete filler or something? Or should try to get a couple of really small pieces of backer board in there? What to do?

Fill it with caulk. If its deep use backer rod and then caulk over it. Tile to within and 1/8" of an inch of the tub and then caulk that joint as well.:)

Awesome! So I should tile to within 1/8 even though it means that about 3/8" of the tile will be hanging over the caulked space where there is no concrete substrate. It will be ok for just that little bit...

I can't see it but sounds like something you could just fill in with thinset as your working and then caulk it.