Owen Duffy wrote in
:

On first glance, the change in peak anode voltage and current is
indicated on the anode I/V characteristics by laying an incrementally
different load line on the chart and observing the change with peak
grid voltage held constant. The deltas then could be used to calculate
a dynamic source resistance at the anode. Essentially, the value
calculated will be the inverse of the slope of the constant grid
voltage line.

The last sentence should read:

Essentially, the value calculated for a class B amplifier will be about
half the inverse of the slope of the constant grid voltage line.

Owen