Take readings when the system is in a steady state condition, usually after it has been running for 15 to 20 minutes. Make sure the unit stays on during testing by setting a low temperature on the thermostat. If the system temperatures and pressures are no longer noticeably changing the system is at steady state.
Best results are obtained when all temperature and pressure readings are recorded within a short period of time. Varying outdoor afternoon temperatures can change enough over a 30 minute period to significantly alter your target values.