Tile edging above thick concrete

[alexRDIY]

 

Hi we are sprucing up our bathroom. And currently the tiles only come halfway up the bathroom and have been installed crudely. We have bought some tile edging the same length as the tile to make it look more finished. However there are varying levels of concrete thickness on each wall which the tiles have been adhered to. The thickest concrete being 1.2cm.  Any tips on how to address this? Should we put anything behind to compensate for the gap or just broach the gap with the same amount of adhesive?