self leveling cement questions

gramps416

I am in the process of renovating a bathroom, using ditra and kerdi
Can self leveling cement be used in a very small area in the bathroom, or does it have to be poured over the entire surface?

There is a valley where the bathtub feet used to be. Over time, it deformed the subfloor.

From what i understand, self leveling is a difficult process. If there is a slight peak or valley, what is acceptable to warrant self leveling? Let me explain. If a floor sloped 1/16" of an inch over 3 feet is that considered acceptable? Is 1/8" of an inch. what is the threshold of whether to attempt self leveling or not?