What is the correct mortar mix to use for a pre-slope portion of a custom tile shower floor?

The preslope and setting bed are the same for the most part. You want to have 1 part portland to about 4-5 parts sand. I prefer a richer mix and like it more on the 4 parts sand. If you purchase bags of sand topping mix and bags of sand, the sand mix is 3 parts sand to 1 part portland. 4 shovels of sand mix to 1 shovel of sand will give you the 4-1 ratio. It gets mixed with only enough water to make it a packable consistency, like if you were making sand castles.

Now here is the exception to the rule... If the drain collar is set flush with the subfloor, then you will have a very thin layer of sandmix near the drain for the preslope. IF that's the case, you will mix your preslope with latex instead of water. The stickiness of the latex makes it trickier to shape. If the base of the drain where the screws go into the base are resting on the subfloor like feet holding it up from the subfloor, then you can use just water. For your setting bed, only use water.