I think what really matters on discharge, is the load on it when it gets low. As you get down towards 10%, internal resistance goes up, and the same load it handled nicely when full will make the cell get hot.
And as said above, heating up the cells is really the thing you need to avoid.
Charging to less than 4.2v can make sense for other reasons, like not having to have the pack perfectly balanced. I liked to charge to 4.15v, so I could more safely use a dumb bulk charger. But still, even at 4.1v, good idea to limit the time it sits full, to a reasonable degree of convenience. I liked charging to about 3.9v to store overnight, though that is still a bit high. When I needed them, then I could top up quicker than if I stored at 3.8 or less. If not expected to use for a week though, I'd leave them at 3.7 or so, where I stopped. If really low, I'd top up to 3.7 or so.
A pack with a bms though, it might be worth it to just charge it full, and let it balance, then store. Particularly for a long storage, where a bms might kill one or two of the cells that power the bms.
Whatever voltage is 30%, lowering the load when you get below it is good thinking. What is that voltage? that can depend on your starting voltage, whether due to age and inability to hold 4.2v anymore, The load on it, or just what you charge to in the first place.
You can find any percentage by first testing total watt hours. To make the math easy, lets call it 1000wh real world capacity, by test. Discharge out 700 wh, and you have 30% left in this example. Turn off, and let voltage recover, like for 30 seconds or so. now you see your 30% voltage. Apply load again, and now you see your 30% voltage under that load.
Other than as said, don't overheat your cells, and you can stop worrying about what the voltage is under load. But as that discharge gets very low, like the last 10%, keeping the cells cool means a discharge that does not sag much.
30% stopping point is not some magic number that saves a battery imo. But if your load is constant, then that may well be the stopping point that avoids heating up the cells, which definitely kills them. how fast depends on how hot they get.
Bottom line is simple, keep em cooler, by varying discharge rate as they get low.