Yes, half of the x-height is used:
This is alluded to in the Handbook in the aligning section:
If the italic angle is not zero, instead of the left sidebearing, an imaginary vertical line crossing the slanted LSB at half x-height is used. In that case, the origin point is where this line crosses the baseline.
Note that a font may not have an x-height metric. I am unsure what the best value is then. That might also depend on your plugins use case.