ST1S10 step-down regulator - can I drive the EN pin from the voltage divider?
0.4V and 1.2V are just the guaranteed limits. The typical threshold is around 0.75V, but it is strongly temperature dependent (see figure 12 in the datasheet). 

You need an external comparator and voltage reference. Depending on the input voltage range and detection threshold required, a voltage supervisor IC might do the job (eg. TC54VC). 
However you may find that the regulator turns back on when the load is removed and battery voltage 'bounces back', in which case you will need a circuit with more hysteresis or a lockout function.