Tenant Complains AC Not Cooling to 68 degrees, Outside Is 114 degrees

[Chuck E.]

This issue has occurred several times at different properties: tenants complaining that the AC isn't cooling to a ridiculously low temperature when the ambient temp is about 40 degrees warmer. My response has been that a professional does preventative maintenance annually and confirms working order of the AC. Upon receipt of these complaints I'll personally check the temp splits ("delta T") in several rooms to confirm they are still within the acceptable range. If the tenant is still not happy I let them know they're welcome to purchase a window air conditioner so they can attain their desired living conditions. 

The AZ Landlord Tenant Act doesn't specify a mandatory minimum temp that must be attainable. It only states that "reasonable amounts" of "air-conditioning or cooling, where such units are installed and offered" must be provided (ARS 33-163). Well, I'm sure you can agree that 68 degrees is very unreasonable with an ambient temp of 114. I've received another complaint that the temp won't reach 74, but this was on their move-in day when the thermostat was at 82 the previous day. Obviously it takes time to reach such a low set point on a very hot day.

Here's where I'm looking for advice: What is the lowest reasonable temperature to set the thermostat in the dead of the summer? (I keep my house at 81 degrees so I feel rather apathetic to these issues.) Any advice on how to cordially respond to these 'work orders'?