I'd say that Conqueror's true damage conversion should scale off the difference in Maximum Health between you and your target, like the Cut Down rune. Start at 10%, scaling up to 20% based on the difference in Maximum Health.
I'm not sure what good numbers would be for this, though, since a lot of the big Conqueror users get a bunch of Health on their itemization, to where their Max Health won't be that different from the tanks that Conqueror is supposed to help them win trades against until several items in.
Also, the true damage conversion should be adaptive, only affecting your primary damage type. Jax and Irelia already have a bunch of mixed damage, and mixed damage is already harder to itemize against. Conqueror shouldn't be doubling down on that.