Not quite... you don't track damage after you hit zero. Any damage causes a failed death save only (crits cause 2). The only time you check the damage dealt after zero is to see if the damage dealt in that hit exceeds hit point maximum. If it does, you suffer instant death. So, the only thing the Zealot in your example has to fear is being hit for 150 damage in one go. Otherwise, she can soak an unlimited amount of damage.
OK I was unsure about the damage after reaching zero. The death save part does not apply due to the ability states it ignores that rule.(unless of course you were using it as an example of the normal rule.)