You need 2k armor because in Torment 4 you get -1000 armor.
If you really want to calculate your armor values by hand, you need to get to 2k armor, because all multiplicative armor stats will work before the T4 malus is included.
Total armor = additive armor values * mult. armor bonus 1 * mult. armor bonus 2 * … * mult. armor bonus k - Torment malus
You see, that the Torment malus is subtracted at the end?
If you have 1020 armor in T4, you will have exactly 2020 armor in normal difficulty.
- Let’s say you need some more armor on your character
You put % total armor as a defensive tempering on your item which rolls between 10.5 % and 15 %. With 12/12 masterworking its getting up to 15.2 % on the lower end and 21.8 % on the upper end, both without crits on the masterworking.
Let’s say I only need a 15.2 % roll to get to 2000 armor, 1000 armor in T4.
2000 / 1.152 = 1736, which is 736 armor in T4.
I looked it up for 740 armor and its damage reduction is only 52 % versus the 85 % from 1000 armor.
So instead of 15 % physical damage you get with full armor, you now get 48 % damage, that’s 3.2 times the damage.
That’s a reason some players get one-shot in the game if they are a bit careless when crafting. - Ultimate cooldown reduction was also mentioned.
You just cannot play your build if your cooldowns aren’t ready in time.
Sometimes you have breakpoint where you need to get to a specific threshold to make playing the build smooth. - Maybe you are just using corpse tendrils and you want them at their max radius (+100 %) and you need 2 temperings for that, so with only 1 tempering you will only get +50 % radius.
With the armor example, you cannot even play this character in T4 without full armor. When you plan your build, you will never plan with a big surplus of armor because it will weaken your build overall.
You could use the item in T3, but you would prefer one with 2 good temperings on it even with only 1 GA.