Nevada will be an Easterly declination so it will be the opposite from above.
Easterly Declination: Mag to Grid - ADD. Grid to Mag - SUBTRACT.
Westerly Declination: Mag to Grid - SUBTRACT. Grid to Mag - ADD.
I'm relatively new to DCS and have mainly flown in the Caucus map so far, anyone please correct me if I'm wrong. So far my understanding is that the map in the mission editor and the runways are True North but the aircraft report magnetic headings in their instruments like HSI, etc. I'm not sure if DCS World is modeled after real world declination or not but Nellis AFB is currently 11.8 degrees Easterly declination, so 281 (Grid) from the map in the mission editor minus 11.8 is 269.2 (Mag). Close but not exact to 267 still unless the ruler lines in the mission editor were roughly drawn to get 281 degrees causing the difference.
One last thought, magnetic declination is constantly changing, so does the declination in DCS stand still in the time when the map (Caucus or NTTR) was created or is it being updated to match the real world declination every year?