The POH should tell you at what altitude full throttle will produce 75 % power undre ISA conditions. Any takeoff at this or higher Density Altitude should be done with the mixture leaned to max power. The easiest way to do this is at the end of the runup apply full throttle and then lean to max RPM (assuming fix pitch prop and carburated engine) and then slightly enrichen the mixture (move the lever about 1/4 the distance from full RPM to the full rich position). Use this setting for the takeoff. Regardless all hot and/or high operations require carefull monitoring of CHT and oil temp. As a very rough rule of thumb most engines will produce 75 % power up to a Density Altitude of about 7000 feet.