refer to my other post on how I got to this point:

So, I cut the cast iron before the trap, and I am going to pvc with a fernco rigid adapter. Check picture below (pic is before I cut pipe). I have a oatey glue on pvc drain where the top screws out and it has 2 gaskets. Here is my question:

I am planning on re-centering the drain in the middle of the spot so I can use a sterling 48" pan now. I plan to set the top of the glue on piece about 1/2" above the slab floor. Then I plan to use concrete to fill in the hole and come even with the top of the flange. I then assume that the new pre-formed acrylic shower pan would simply sit on top of the flange, with gasket, then the top screws in, inside the pan. The manufacturer says to screw the pan to the studs and use concrete or mortar to set the pan.

Does this sound correct? Or should I set the glue in drain, only use the concrete to fill the hole, then use mortar to build up the bed to meet flush with the top of the flange?