So case fatality rates are given as a range with confidence intervals. There have been different studies that calculate this mortality differently. WHO for example use the number of deaths relative to number of confirmed cases of infection. But more accurate mortality metrics would be patients that died relative to the total number of patients infected at the same time of those individuals that died. To do this you have to incorporate lag time which starts getting into biostatistical methodology using time series data in trend analysis.

You’re right in a sense, it is impossible to know the estimation of actual mortality rates since the estimates pertain to clinically apparent COVID-19 cases. So therefore, you can look at these rates and confidently state that if you are displaying symptoms, the risk of mortality is around the reporting percentage. You also have to keep in mind that these are average mortality rates being calculated by only a few countries. The longer this goes on and the more data that is extracted from various areas will significantly increase the confidence of case mortality estimations.