The Bf 110 G-2 manual says that the Bk 3,7 was zeroed at 800m. It should be hitting at the reticle centre at 340m and 800m, and 42cm high at 600m.
Manuals available here
The relevant graph is in 'Bf 110 G-2-R Teil 8 A.pdf', page 30
Using the bottom of the sight ring works ok in game but it would be better having it as they originally used them.