The reason I said 40-60% is because over that entire range both self-discharge and permanent capacity loss happen at their slowest rates because that is the flat range of the voltage curve where the cell is close to its nominal 3.7V voltage. The self-discharge with starting at 80% will maybe buy you an extra couple months before the cell becomes unusable, but you would experience more irreversible capacity loss.
The reason I said 40-60% is because over that entire range both self-discharge and permanent capacity loss happen at their slowest rates because that is the flat range of the voltage curve where the cell is close to its nominal 3.7V voltage. The self-discharge with starting at 80% will maybe buy you an extra couple months before the cell becomes unusable, but you would experience more irreversible capacity loss.