4 foot above grade is what I remember: 42" guard rails, toe boards (4" x 1/4), mid-rails, and anti-fall-back guards (chains or flip-type guards) at all entrances where a ladder lets you go through the rails.
Count user needs too: not just what "the law" calls for: You'll need a place to rest eqpt, lay down access gear, stage supplies, open doors or hatches, possibly a rail or hanger or rigging point for the door (if heavy), another rigging point (or three or four) for lifting stuff in and out of the tank, a tank watch space (if hazardous gasses or enclosed space, ...)
Further up (higher - but I don't remember how far) you need the full ladder cages and have to start counting the rungs before a safety stop.