Root zone temp is like asking what the best temp for ur plants are. There are soooo many variables in determining what that should be such as: co2 supplementation, room temperature, whether ur using soils, drain to waste soilless, or hydroponics. Root zone temps ideally considering ur ambient room temps are correct shud fall somewhere between 67-73 degrees.
Under 65 and nutrient uptake is diminished, over 75 and pahtogens and bacteria begin to take over. If ur room is upwards of 80+ then u need to lower root zone temps and lower ur watering temps.
For example if ur room is 85 ur gonna wanna start dropping ur water during feeding temps lower to combat the higher temps. When im running hot during summer or co2 injection I've water in the low 60's with no issues. I wouldnt go lower than 60F ever as u can cause issues such as lock outs and deficiencies bcuz when temp gets too cold in rhizosphere nutrients arent able to be broken down and absorbed properly. BOL