I would say Fairfax is probably on a competitive scale I would put it right up there as one of the most competitive, mainly because you have a lot of military in the area. The upshot is because many are military, some will come in on Presidential nom besides the MOC noms. Additionally, since the schools are academically reknown, even on the national level, I.E. TJ has been for yrs the number 1 public hs in the nation, many of the cadets also come in from the national list.
I recall one of my friends saying that in her son's yr, out of the 20 on the 2 Sens. list, 18 got appointments.
Now the sad news, I believe she said there were over 300 applicants for the Sen. noms., and @75 for the district. If you have a strong gpa and SAT, with strong ec's you should be fine.