Not sure what an acceptable mortality rate would be but I do know that you can minimize it by letting the bag of fish float in the pond till the temperature is the same. Quick changes in temperature can really stress fish and make them more suseptable to parasites and infections. Next once the temps are the same in the bag water and the pond mix some pond water into the bag wait a few minutes and then mix in more this is to get them acclimated to changes in water quality. These steps will help reduce stress on your new fish. keep us posted to how many you lose.