I fly electric planes too, you can tell when the power is draining, and most ESCs cut the power at low voltage in any case. I usually dont fit LVC alarms here.
I fly small hand launch gliders too, I do fit LVC alarms in these since I want o know if I am going to loose control of the plane because the battery goes flat, before it actuall does.
For planes flashing lights might not be that visible, check on eBay and you will see that the LVC alarms you can buy use piezo sounders. Tuned to the right frequency these are very loud and can be heard in flight.
As for the potentiometer, you can fit it, adjust the LVC alarm point, and then work out what combination of resistors will give you the same result.
Also be aware that the ESC may not be putting out 5v, the ESC on one of my planes puts out 5.3v for instance.