how reference a translation of an already translated text? Somebody wrote a text in Latin, let’s call her Alice and let’s call the text De Nihilo. This text was then translated into German by Bob. In my work, I translate Bob’s translation into English.
In the author–date system of the CMoS style, I have adopted this solution for referencing a translation of an already translated text:

My own translation of the following translation from Bob. (Bob 1987; translation in German of De Nihilo by Bob).¹

¹ “[Latin Text]” (Alice 47BC).

Is my solution the correct one?
STEM texts

In STEM texts, the main goal of providing the reference is to give attribution to ideas. Thus, the reference acknowledges that Alice wrote De Nihilo in 47 BC.


  As Alice has pointed out long ago (Alice, 47 BC), nothingness does not exist.  ...  Alice, De Nihilo, 47 BC, Rome, Manuscript Publishing.


The secondary goal is to point the reader to the available translation, in case he wants to verify the source himself. In your case, this is the German translation Bob made.


  As Alice has pointed out long ago (Alice, 47 BC), nothingness does not exist.  ...  Alice, De Nihilo, 47 BC, Rome, Manuscript Publishing = Alice, Über Nichts, 1987, München, Hanser.


No one cares if you made your own translation into English; readers value ideas first.

Language studies

In language studies, you are likely interested in how exactly the same idea was expressed, maybe to the point where you are pointing out differences between several translations. Alice and Bob deserve separate mentions now. If you have taken the effort to translate De Nihilo into English, this deserves to be published, say, in appendix A of your work.


  Alice is genuinely worried by the nothingness conundrum (Alice, 47 BC), but Bob's German translation fails to convey this emotion (Alice, 1987). My English translation (appendix A) seems to resolve this problem.  ...  Alice, De Nihilo, 47 BC, Rome, Manuscript Publishing.  Alice, Über Nichts, German translation by Bob, 1987, München, Hanser.