So I haven't read this thread in detail so apologies if I repeat stuff:
- we can't tell if it's under-exposed because this seems to be a negative scan and there's endless room for interpretation there*
- there's detail in the dark shadows, so the exposure seems to be perhaps sufficient
- this is Portra 160, you can nuke it until it glows (+3 stops) and it will scan or print fine; don't be afraid to use more photons
- I too question the choice of hair for metering
- since she's light-skinned, spot-metering to put her face at Zone VI (+1 stop over metered exposure) will look about right**
- to me, a backlit portrait will deliberately blow the hair highlights and use of fill would reduce the backlight drama IMHO

* it could be beautifully exposed according to all the guidance in this thread and the scan operator could make it look like you've posted
** I think this will result in about 2 stops more exposure than you used according to the hair