Aren't you by chance using a Pentax 67?
Suggest a back-to-basics metering strategy:
Spot meter face (1: in this image the face is a critical fail as it is improperly illuminated), left arm (illumination side) (2) right arm (3), front of dress (4) forward bottom left grass (5), then average all. No spotting of spectrals in background or bright grass.
A flash should not be needed but can be usefully employed with spot metering and baseline-averaged fill-in flash with the softbox you spoke of.