[Gert-Jan]

Certainly there are more sophisticated methods to calculate the mixing time in Post. But if you can't measure this in practice, it has less value. Also, these all remain derived quantities. In principle they should all lead to more or less the same answer. Therefore I would remain comparing the mixing time from the monitoring point as monitored in the experiment. That is what you have in practice.

However, it is not said that this point is the most valuable, characteristic, or critical. Maybe you can find a location in your CFD that is more critical in your chemical/biological process? E.g. look for a region with still water conditions. Then I would advise to move the sensor to that location.

That is maximizing the power of CFD. You open the black box.