Generally over 250 degrees is your limit on trans temp as that starts to degrade the oil. Synthetics can take the heat a bit better, but they still have a limit. The longer the oil is at that high temperature the higher the chance of eventual failure. I would stop and idle the engine in Neutral for 10-15 minutes to try to cool it below 230 before moving on.
On all my vehicles with Auto transmissions, I try to keep them under 200 on normal type days (80's). Even added a larger cooler to our RV Allison transmission when it started running 230 on easy roads in the 80's and low 90's.