Usage telemetry can be an effective tool to assess the value and impact of an open source project, and enable time-strapped maintainers to prioritize support for features, versions, and/or components that are actively used. However, many open source users see incorporating usage telemetry into a project as an invasion of privacy, especially for projects that did not launch with embedded telemetry. It is therefore important to have reliable proxy metrics for many projects and companies that need to show the value and impact of open source projects. In this proof of concept, we use Flutter as our test case to compare proxy metrics generated from publicly available sources with Flutter’s actual monthly active user count collected by embedded telemetry during the same time period. Our results show that cumulative Fork and Star counts across all of Flutter’s GitHub repositories have strong positive correlations with Flutter’s monthly active user counts. Our conclusion is that some publicly available metrics may be suitable proxies for usage metrics.