A timestamp is not a fill
An alert can be published before a move yet still be impossible to execute at its stated price. Log the time received, bid and ask at that moment, order type, market depth and your actual fill. Compare against the original unedited message, not a later recap.
Use the same convention every time
For long alerts, a fill above the planned entry is worse; for shorts, a fill below it is worse. The worksheet expresses that drift as a fraction of the advertised entry-to-stop distance. A 25% risk-budget loss to delay is not a 25% account loss; it is a warning that the published trade geometry changed.
Test the distribution, not one anecdote
Record a consecutive sample of calls, including those you missed. Calculate median and worst-case drift, and separate liquid sessions from event-driven gaps. If the provider changes entry targets after posting, save both versions. The service's headline hit rate is irrelevant if its executable entry consistently disappears before subscribers see it.