I've always based it off of heat index, not temperature alone, nor humidity alone. It can be 90% humidity and 30 degrees F out and I will certainly ride. But 90% humidity and 90 degress, no way!
I always learned a heat index under 120 is fine, over 150 is a no go, and in between depends on the horse, rider and situation. But that is a very general rule of thumb. You must take into consideration your horses fitness level.
BTW, heat index is degrees Fahrenheit plus the relative humidity.