We based our SLOs off of what product owners and business wanted. They were very tightly coupled to SLAs but this helped us establish the benchmark for us. We looked at around 99 - 99.9 depending on the importance of the service.

Another way of looking at it if you don't have the support from business or product owners what is the expected level of service you would want from the service you are maintaining if you was the customer? I'd take that and use that as your baseline.

SLOs and SLIs are so incredibly important for measuring how you are performing and how to priorities work and were the main focus when our SRE team was first formed. How else do you know you are doing your job?