Once the gap is clear, resist expanding the work into a complete onboarding redesign. Fix the part that prevents the intended result, and preserve a way to see whether the change helped.
That could mean explaining the origin of the data, making the next action available, preserving a required field or showing an actionable message when an input is missing. It could mean changing the promise made before signup if the product serves a narrower situation than the customer expected.
Agree what the next observation should show. The customer should be able to complete the task and recognise what to do with the result. If you intervene, record the intervention rather than counting the session as independent success.
For an early product, a small set of customer notes can be sufficient to begin. Record the task, whether the customer had a real opportunity to perform it, the point where progress stopped and what happened after the change. Numbers become more useful when the underlying event has a stable meaning.
Keep setup measures as diagnostic measures. A broken connection still deserves attention. The improvement is to connect those measures to the customer's task instead of asking them to carry the whole definition of value.
Finally, give the check an owner. A developer can implement the change while the founder returns to the customer to understand the result. If both people assume the other will follow up, the startup can ship the fix and end up with the same unanswered question.
Consider what would make the milestone misleading after the change. A task event might fire even when the output is empty. A founder may complete the action on the customer's behalf. A customer may use sample information that never enters their actual work. Those observations can help test a path, but they should not be silently combined with useful customer outcomes.
Keep the definition understandable to the people using it. If the founder calls a customer activated because setup is complete while the developer assumes activation means a usable output, the same word will support conflicting decisions.
A short shared definition can state the task, the conditions and the remaining uncertainty. That is enough to make the next customer conversation and the next product change refer to the same finish line.