Barry Klein wrote:

How should we have found the correct mixture setting
for power-off final approach, when the turbo is basically adding no
boost and we are at high density altitude?

Did you enrichen the mixture between cruise and landing?
There should have been no reason to. As you reduce altitude and MP, the
mixture will enrichen with decrease in altitude. Had you need to do a go
around, the cruise mixture would have been sufficient for a full power
go-around.