You can do a 80% wc and not harm your fish if done correctly.
Match the exact temp of the tank water with the incoming water with a accurate IR thermometer
BTW F1Vet, in an area where tap water values such as pH, TDS, etc can change throughout the year, IMO your previous comment posted above would be very bad advice for people new to this hobby. Just like nitrates, temp is only one value that one needs to be concerned with if doing very large (80%) water changes.
Lucky for me, I don't have that type of issue as our local water values are constant year round.