You would use a straight edge to screed over the 3ft. width. It will look ugly and rough. Have a wet sponge or spray bottle with water on hand. Allow it to set up a little, use a 18" steel trowel to smooth and flatten it out, feather the edge as needed. If, it becomes too hard to work with, spritz or sprinkle a little water to soften it, then work it with the trowel. You can use the steel trowel to scrape and carve out unwanted high spots.

Follow the curing instructions before installing flooring over it to avoid moisture damage.

"can I taper the application of the self-leveling cement?"

No. What you should use is a trowelable product, not a self-leveling product. You mention "feather-finish", which is exactly what you want.

https://www.ardexamericas.com/product/ardex-feather-finish/

Self leveling products are mixed thin so they flow and seek their own level, and are made to level, not ramp. Also, they generally don't finish to a feathered edge.

I'm unclear as to whether you're trying to ramp down 1/4" over a 3' span. If so, that's out of generally accepted standards of 1/8" variance in level over a 10' span. Going up 1/4" in 3' will likely yield a noticeable ramp and may cause issues with the material going over it. Grinding down the upper part so it starts sloping sooner could help, but you should probably figure on feathering the low area out at least 10'.

Thank you both for the extremely helpful information! If I'm using a primer and feather finish by Ardex, Sika, Mapei or Bostik, to correct this quarter inch dip that is 3 ft wide x 26 ft long, and I am allowing 10 ft to transition tje leveling to the height of the existing flooring, is there any concern that the feather finish might crack and / or my floor might fail for some reason? If so, are there pitfalls to avoid when applying the feather finish?

With much appreciation,

Amy

Feather finish from Henry (Ardex) does not need primer coats for adhesion. As long as your surface is nice and clean your fine. The product willl not flake off.

Nothing to add to what G&S wrote.