I am tiling with 600 x 300mm tiles mostly over existing tiles. I have primed ith Mapei Eco Ultra Grip. The lower section of wall where the bath was is not tiled. I have installed 10mm backerboard there but it is around 4mm less proud than where tiled. How can I fill this area in to bring it to the same level as the pre tiled upper wall, before tiling over with new tiles. See photo.

