Joe, I usually assemble the drain and use the top of the drain as a rough screed for the final setting bed. If I am using a wood straight edge, I'll nip off a 16th or so of the inside (drain side), so the final setting bed is the thickness of the tile lower than the drain. The tile thickness determines the depth of the nip. If I am using an aluminum straight edge, which is more likely, on my final passes, I'll dig into the bed at the drain side to make the final slope. Final clean up with a steel trowel and a chunk of tile complete the feather.
If the mud bed is really sandy like mine, even the next the day a steel trowel can be used to shave off a portion to make the slope lower than the drain. I don't know where you are at with this project, but you might consider that.
If the mud bed had additives in it or is really old, that ain't a gonna work.
"Sir, I May Be Drunk, But You're Crazy, and I'll Be Sober Tomorrow"
WC Fields, "Its a Gift" 1934