The ability reads along the lines of:
Relentless (Recharges after a Short or Long Rest). If the [creature] takes [number] damage or less that would reduce it to 0 hit points, it is reduced to 1 hit point instead.
To me, there doesn't seem to be an explanation for the amount of damage to avoid triggering the effect. This creates a problem when I want to give this ability to a custom creature.
Are there guidelines I'm missing? If there are not, how do I know what number to use there?
The creature(s) I'm homebrewing are Swine NPCs (SwinePCs) that possess this ability, and their final CR ranges from 1/4 to 5. I know I can just use the numbers from the Boar and Giant Boar for the same CR levels, but I'm stuck with the CR 5 creature.