Pterist:
This is the information that was available to Daniel, since he was now in exile almost 70 years (NOT 90), he prayed
This conclusion is based on an inaccurate rendering of Jeremiah 29:10 that is not supported by the context.
There was never any mention of 70 years of exile. Daniel* realised from Jeremiah's writings that 70 years of Babylon's dominance had ended, and he knew that Jeremiah said God would then 'turn his attention' to the Jews going home. So Daniel then prayed to that effect, realising that in Darius' first year, it was now time to arrange for the Jews to go home.
*The historicity of Daniel is disputed, and the book is generally considered to have been completed in the 2nd century BCE. Regardless of authenticity, this does not discount that there may actually have been a Daniel on whom some of the stories were based, or that Daniel may have written parts of the book.
There is no reason to believe that Daniel thought, "I'd better start praying now because Babylon was just called to account, and I've been in Babylon for 80 years*, so the 70 years must end in two more years."
*Daniel was taken to Babylon as part of the tribute paid to Nebuchadnezzar in February 604BCE. However, in the JW chronology, he was taken in 619BCE (rather than 625/4), due to their baseless interpretation of 'kingship' as 'vassalage' at Daniel 1:1.