Hussar
Legend
What can you suggest?
I'm not much of a mechanics guy, but, I'd say you'd tie maybe half cleric level to whatever CR it can turn, minimum of 1. Total numbers of enemies isn't quite as important, because, well, let's face it, if you're facing multiple liches, the PC's have bigger problems than how many liches can be turned with one check.

Just as a question though, if you're cutting PC HP and then cutting monster Damage by the same ratio, what' the point? Seems a bit unnecessary. What difference does it make if the PC has 10 HP and the baddy does 10 damage or the PC has 5 HP and the baddy does 5 HP?