Technically it depends on what the manufacturer uses to rate capacity. If you want to test the capacity, put it on a charger, discharge to 3.0v/cell, and charge up to 4.2v/cell. Record how much mah went in. Now if you end up putting in 5000mah then your answer is 3.0v.

The manufacturer may have overstated or understated the value.

It's sort of like you need to actually find out the true capacity and if you do that you have already answered your question.

And I think the true min voltage of a lipo is something like 2.85v and max is like 4.22v. Don't quote me on that. It's best to work with 3 and 4.2. But the manufacturer might have used that.

Nominal voltage is the average voltage under load. If you look at discharge graphs they dip down quickly toward nominal voltage then stay there for a while. Toward the end they dip down quickly again.