I will be building up a shower pan using the Oatey Perfect Slope pre-slope system, a 40 mil liner. and then building up a dry pack mortar bed on top of the liner. My question is what is the best detail for where the cement board meets the pan? Should the cement board come right down onto the mortar bed? And if so, should that change of plane be sealed with some sort of mesh tape before the waterproofing goes on? Or, should there be a gap below the cement board to prevent moisture wicking up through the cement board?
Here are the steps I used for decades with great success (no water problems).
You can use the Pre-slope, but I have used a perfect slope to get the mud bed correct and didn't use the pre-slope.
Cover your pan with the 40 mil liner. Nailing the liner at least 8 inches up the studs. Also wrap it over the curb and nail on the outside only. (If you want to use your pre-slope, install the liner over it, but your mud-bed has to be an even thickness throughout.)
Install the cement board to the bottom of the pan over top of the liner nailed to the studs (all the way to the bottom and sitting on the liner is ok). No tape is necessary. Screws at least 6 inches up from the pan. The mud-bed will push the last few inches tight against the studs. Tape and mud the corners and seams and mud over the screws.
Install the slope system of guide sticks. (If you install the slope system here you will be most assured you will have a correct slope to the drain.)
Install the mud-bed guided by the slope sticks. Pack it well.
When dry waterproof the mud-bed and the walls with RedGuard or AquaDefense or similar. Apply 2 good coats. (Pre-slope now is moot.)
This makes a waterproof shower, even if you didn't add tile (but you will). Wicking, seeping, leaking any of those problems will not manifest themselves if you did all the steps correctly.
A final note: Some prefer to seal the grout, some do not. I only seal if the covering is a type of porous material like stone that needs to be sealed, or if the grout lines are wider that 1/4" which is rare anymore.