Just a small observation. The terms 'positive and negative' are usually used when referring to AC.

When dealing with DC I've always understood 0V (Nought, or zero, volts) to be the 'negative' with either a plus or minus volts being referenced to it.

A split rail PSU would be be a twin rail supply with both a plus and a minus volts referenced to 0V.

Therefore, when considering a minus volts, the 0V (generally referred to as the 'negative') would in fact be the positive.

Therefore, imo, it's always best to refer to DC voltage as plus volts and 0V or minus volts and 0V, not positive and negative.