I have to build a 40mm platform and I have two questions. The first is how deep should the sand/cement bed be, its a stone/ resin tray? The second concerms the width or land of the platform outside of the tray, I want this to be around 50mm max. and intend to tile it. Should I bring the bed out to the edge and just tile over it or make an edge to contain it and tile over this and the bed?