I dont remember the ehp calculation but I ve found this,
https://rehfeld.us/wow/effective-health.html . At 50k hp and 40k armor, 1 stamina gives 37 eh, while armor gives 11 times more and getting 900 armor at that point increases eh from armor by 0.2 I think. So 60 stamina gives 2220 eh compared to 6549 eh from 885 armor (885 x 0,2 = 177stamina and 177 x 37 = 6549 eh.) And about head enchant, its just +8 stamina since rep enchant already gives +37 stamina so its not that great, I wouldnt pass +20 def rate at expense of 8 stamina even though Im never gemming for it.