The periscope reticle in the PV1B does not have a slew cross hair like the reticle in our 234. The image below shows the reticle for the periscope used in the Ar234. The second one down would be used for the 20mm and point to an aim-point viewable above the stab.
(Image removed from quote.)
Here is the reason and why it never worked.
So this is kind of what you would be looking at with an enemy aircraft on your six.
Think about the control surface inputs you would need to fly the plane onto the target at your rear.
Plus looking though a field of vision like this actual AR-234B rear view of the recon version periscope.