What is the perfect truth in Tientai Buddhism? It it well known that Tientai teaches the identity of the mundane and real truths.


In its complete teaching, what is being equated?


Is it equating the real truth of the separate teaching with the complete teaching's mundane truth?

The so called "separate teaching" is what Yogacara and Hua-yen Buddhists claim, and Zhiyi says (translated by Swanson) says that therein:
I'm going to answer based on a hunch. Swanson notes, in his translation of the Mo-Ho-Chih-Kuan, that


  These three [truths as] expressions [of the perfect teaching] seem to
  be the same as the formulation in the Distinct Teaching, but as
  Chan-jan points out (BT–I, p. 284), the Distinct Teaching takes each
  of these formulations to be a separate level of understanding.


My hunch is that:


for the separate teaching, the mundane truth does not contain anything of the
absolute, and is only its negation.


As the perfect teaching merely integrates the three truths of the separate teaching (see above quote) it only teaches:


neither existence nor emptiness, and not neither existence nor emptiness.


And, because the perfect teaching includes both truths of the separate teaching, the perfect teaching must include: 


existence and nonexistence


All three truths in perfect teaching, beyond words.


  The three types [of truth such as "emptiness"] are all empty because they are beyond verbalization


My second hunch is that:


we can only express what is beyond verbalisation to say it is merely provisionally true.


Then the perfect teaching must only state that 


any proposition that mentions existence, or non-existence, or
emptiness, or non-emptiness, is merely provisionally true.


Whatever, that is, "existence" is translated as.