Upon further delving, the point where avoidance's utility surpasses Stamina's is about 64% total avoidance if you ar at 22000 HP, at which point you are always better off stacking more Avoidance than Stamina. However, this also assumes a 1:1 trade off. Given that tanking gear in WoW never focuses solely on Stamina or Avoidance, you rarely face an issue of which point you want to grab next, so much as which piece of gear offers an upgrade.