You forgot base chance to be missed which is 5%. The Night Elf bonus is 2% to be missed.
For a Night Elf Death Knight your avoidance against a raid boss (level+3):
GetDodgeChance() + GetParryChance() + 7 - 1.8
Or you can use a variant of my automatic level scaling macro for unhitable for just the avoidance portion.
Night Elf, Death Knight
Target something and this will tell you your total avoidance against a non-player. For example if you target a raid boss target dummy it will automatically do the 1.8% reduction for the 3 level difference. If you target a level 87 trash mob while you are level 85, it will automatically do the 1.2% reduction in avoidance to return your actual result. If you were to target yourself it will calculate your avoidance against an equal level target and have no reduction for level difference. I've already set the miss portion of the function to 7 to account for the Night Elf racial.
/run u,t=UnitLevel("player"),UnitLevel("target") if t==-1 then t=u+3 end ld,d,p,m=0.2*(u-t),GetDodgeChance(),GetParryChance(),7 DEFAULT_CHAT_FRAME:AddMessage(format("Lvl: "..t.." Avoid: %.2f%%",d+p+m+ld*3))