Don't think of it of what you should and should not own. Think about what makes sense to the customers you are focusing on. Then think about where you are in the customer/product lifecycle and product/market fit. What does the overall business care about at this time (e.g. retention vs. acquisition) and start seeing how your product area could contribute back to that overall goal.
Such a great question! When you first set a KPI especially if you are in a new market and/or in a new product/customer space, it can feel uneasy. The best way I have learned is by setting something and tracking it over time, seeing if there is any measurable change. If not start by marking out the customer's journey (no matter who they are) and see if you can collect data on their interactions along the way. This may reveal some hidden trends you weren't yet measuring.