How does this unit behave at lowish battery? The on screen indicator says battery is 50%. I notice I have been overexposing Portra 400 by 1/2 to 1 stop. It could be my film processing workflow, or shutters a tad slow. It could also be the way I am metering.

Ignoring the other things for the moment, does this unit, or other Sekonic meters, change behavior as the battery indicator drops?

Are these meters calibratable? That is another variable in the mix.