You can describe what you did without inventing a measured outcome. Not knowing the result is different from knowing that it failed. Your actions, responsibilities, and observations may still provide a useful and honest example.
If you introduced handoff notes, explain that you created them to communicate unfinished tasks. If a coworker said they helped, attribute that feedback accurately. Do not turn it into a percentage improvement. For future activities, consider one simple way to observe the outcome so you can learn from what happens.