Information about the request calculation algorithm for the REST API is available here, along with the details of how you can measure requests remaining or used after every request.
Additionally, you can query report data related to the usage on your API key for REST API here and for the WebSocket or FIX API here and here.
Have in mind that reporting endpoint should be used only for visualization, analytics, and reporting. You should not use it to decide if you are approaching the limit in real-time as the data in this report is aggregated daily, and the request credits in real-time are calculated from the last 24-hour using a sliding time window. You can find more information about real-time limits here.