Less than perfect is talking about something that does not have a specific quality. So in regards to that quality it is not perfect at all. So another way to say your examples: 1. Her greeting was unenthusiastic. 2. Her advice has frequently been unhelpful. 3. Enjoy your day to its fullest. If anybody in the office makes your day unenjoyable, go and talk to them. 4. My behavior was unacceptable. (Depends on the situation. Needs more context to be changed.) 5. But Cristina was about to discover a situation that was not that great. (Depends on situation. Needs more context to be changed.)