Set the value?
I didn't give the OEB much though as a survey of our engines indicate the margins were well above our actual margins and with an eye on efficiency, well above anything we would have let the engines approach before addressing.
Still though, from what I recall - there's nothing to set.
The margins are monitored by your engine folks. Engines come out of overhaul and operate at a certain EGT specific to the conditions. Over time, as the engines wear/age, the EGT for the same conditions increase to the point where you get to the EGT margin deterioration mentioned in the OEB.
The only action I see would be for your engine folks to advise crews which engines are susceptible to the problem.