I would think that the minimum impedance is the actual trace impedance on that layer when matched. I set the diff rule to 80 ohms.
The clocks below were set to 82 ohms differential but HL DRC is displaying 92.77 ohms on the outer layers. I read somewhere, in the information, that solder mask is not included although I loaded a stackup which does include solder mask in the layer definition. Anyway, that would only increase the impedance by about 3 ohms. Why is it reading high?