I am going to install a brand new shower in my bathroom. It is going where the bath used to be. I would like to know how to install shower tray. I am buying a low level tray with no upstands. I am going to tank the plasterboard walls then tile when the tray is installed. I know that the tiles have to eventually sit on the top of the tray edge then I silicon seal but do I push the shower tray so that is it flush with the plasterboard or do i push it behind the plasterboard so that it is flush against the block wall behind?