Do you mean 12,8 cm Flak 40 or 12,8 cm Flakzwilling 40?After german wartime technical documents -3° is correct.
A different document gives the Z.F.20 E as optical sight for direct fire but I have never seen it on a 12,8 so far.
I've never seen a socket for an optical sight on a Flakzwilling, while it is clearly present on the 12,8 cm Flak 40. I also think, layers were just sitting too low to see anything over the Flakturm parapet.