In the processes of re-tiling the surround of a bathtub in my condo...I replaced the green drywall with hardibacker brand board my issue is on the wall where the shower head and fixtures are there was a "party-wall" two pieces of drywall sandwich together. To make up the difference to the tub I used plywood however I am still about an 1/8th of an inch shy of the tub and its metal "flashing". This also leaves a half inch between the backerboard on the back wall and this wall. So my question is what should I do, I was planning on just caulking the 1/8th of an inch gap and laying the tile over the flashing. As far as the 1/2 inch gap I was going to fill it with thin set. ????