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I have done some experimentation and believe that it IS the minimum output delay that should be negated. This is something I was told when I first start using TimeQuest. I am not sure why but I have given up trying to understand this long ago and now I just accept it.
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I think in terms of tsu/th/tco/min tco and have not developed an intuitive sense of how set_input_delay and set_output_delay min and max work. After figuring it out once to satisfy myself that references that I use are correct, I just rely on those references.
A particular reference that helps me is in the Quartus handbook, Volume 3, Section II, Chapter 8 (in the QII 8.0 handbook), Table 8-4. This table says that minimum tco and set_output_delay are related by "set_output_delay -min <latch − launch − minimum tCO requirement>", which has the negation Graham mentioned. For hold timing using set_input_delay -min and set_output_delay -min, <latch - launch> is typically zero (same clock signal for launch and latch, no multicycle exceptions). That makes the set_output_delay -min value reduce to the negative of min tco.
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