I believe you managed to get depth pass out of position pass till now (and I haven’t seen someone actually answered that question of yours till now), but anyway here it is:
Add Position Pass effect, set it to Camera, chanell order RGB, size to 2. Add Post effects in multipass also. Render as openEXR.
import your footage, make a comp…set your project to 32bit. interpret your openEXR to preserve RGB. Precompose your openEXR, create black solid in that precomp (set it to black, 0,001) and transfer mode to multiply. Apply curves to your openEXR and play with it until you get that blue gradient.
Next, apply frichluft depth effect to your footage, precomp being as your source depth layer. In depth buffer set it to blue channel. Set this effect from normal blur to depth buffer in order to adjust it properly with curves in precomp.
So there you go, depth pass extracted and used and it can be adjusted to your will…and, it works with physical renderer.
Hope I helped a bit.