You don't need to have a big lead in enmity to tank, you just need to be able to lead the enmity race throughout the whole fight.

Just ignore all this talk about 1 enmity over or that enmity is useless discussion, you will use Royal authority because you don't need the bonus enmity from RoH and you can always do with more damage. You might use RoH at the start if you are pulling or if some dps are not using their only enmity management.