So part 3 is going from a straight skirt to an A-line skirt. I followed Winifred Aldrich's instructions, as shown in the diagram below (only my back dart was bigger). She adds 2.5 cm side flare, but this gave more A than I wanted, so I took it out again.
I sewed up a muslin. I am happy with the front and side views, but I am not sure if I need to keep tweaking the back. I think getting the dart shape just right will be important. You can see that now the main dart has been moved out from the centre, it is not hitting my biggest point, which seems to affect the hang. I cut these with the CF and CB on the straight grain, but for a 4 gore skirt, apparently a better "hang" is achieved by putting the grainline down the centre panel after the darts are slashed and spread.