Raj has breakfast almost always before 7:45 AM. On rare occasions, he has after 7:45 AM, but never after 8:00 AM. So If he says "I always have my breakfast latest by 8:00 AM" to convey this fact, is Raj's usage correct?
The question is about the use of the word latest in the particular sense of not later than.
According to the answers here, YES, latest in this sense is used in the US, in the UK, in India, and possibly elsewhere.
All other issues are out of the scope of the OP's question.