Sorry, I don't know how to advise you about the water level. But as someone who has flooded his yard too many times to admit to, filling my stupid little fountain, I can't be trusted to remember anything for more than 2 minutes. So I set up fail safes for such things. Your situation was different (let's just say, uh, sure, I believe you about what happened), but here's a tip that works for all scenarios.
Draining a pool, with pump or siphon hose? Affix either just below the surface. For your pump, you can hang it over the side with a rope, tied to a nearby post, or maybe a 2x4 suspended over the pool, whatever. Point is, if your suction port is only a few inches or so below the surface, regardless if you forget about it, or if it continues to drain your pool after you turn it off, the water can only get so low.
If the suction port is at or near the bottom, then returning from anything that might call you away or otherwise distract you unexpectedly, will find you staring at an empty pool... Just takes an extra minute to make sure that can't happen...