Deck mud over a plywood subfloor needs some thickness to survive, this is why the clamping portion of the drain needs to be above the subflooring height. As mentioned, at least 3/4", and an inch if possible. Then, the liner gets installed, then the setting surface. Did you put in blocking and notch the studs to allow the liner to sit flat and not bow out around the edges? The setting surface ends up being a constant thickness, since the preslope is already sloped the proper amount. The details of how to do this can get specific, and if done wrong, you'll end up with a leaky, and/or smelly shower. You want the setting bed to be at least an inch thick, too. This will somewhat dictate how high the curb must be. Many people assume that the tile is the waterproof layer, but it is not, the liner is, and it must be sloped and the weep holes open, or you'll slowly accumulate stagnet water underneath the tiles in the mudbed. This is one reason why I prefer a surface membrane (Hydroban, Kerdi, Nobel, etc.). this puts the waterproof layer immediately underneath the tile, and there's much less that can get wet.