is there a standard or common approach to defining Service Level Indicators?
Whilst not strictly a standard approach, Google has published an SLI Menu and a process for developing SLIs for user journies:


For each User Journey/Data Flow identify from the SLI Menu suitable types of SLI:


Make a decision about how to measure good and valid events,
Decide where to measure the SLI From out of the following: End User, Client-side Instrumentation, Synthetic Clients, Front-end Metrics, Application Metrics or Server-side Logging.


The document then goes on to describe, how you collate all of your SLIs for all of your user journies then walk the journey to look for coverage gaps.  Then finally set SLOs based upon the SLIs either based upon business need or past performance.