there are different cultivars of Tall Fescue. Some have a more dwarf growth habit and can be mowed lower. K-31 is not the same as Rebel. The finer stuff I would mow at the low end of the range, the coarser at the high end, for the particular time of year. Maybe 2.5" to 3.5" this time of year. Lower than 2.5" and I think you may be doing more harm than good long term.
With spring growth, you can mow it lower to keep up. However, with the summer heat you need to protect the crown with a taller cut. The bottom line is whatever the customer wants no matter what. We all know better and people will talk about their reputations but, it all boils down to who pays you.