Building on the answer of LessPop_MoreFizz, to answer the follow up question of whether there are increasing returns. The stated equation for absorbtion rate is:
DR = Ak/(1+Ak) where A = armor, k = 1/(50*moblevel)
Id rather like it on the form:
with lvl being moblevel.
If you do the math and take into account that the raw amount of damage you can take with fixed health(and dodge and other modifiers) is
and you insert Dr and reduce you end up with:
raw = (1 + A/(50 lvl))*life
So if your armour is 50 times the level of mobs you meet, you double your "effective life", if your armour is 100 times the level of mobs you meet, you triple it. I think that mostly fits the description of constant returns, you effectively get as much +vit as your current life for the cost of 50*lvl armour points. This also shows that if you are high on life, you should invest in armour, if you are high on armour, invest in life. Which is a funny mechanic, which isn't exactly diminishing returns, just a case of "better returns elsewhere".
And to answer the question of "If you go from level to level+1 how much more armour do you need to have constant absorbtion?" well, the math works out to
Anew = lvl/(lvl-1)*Aold
So if you go from level 5 to level 6 mobs, you need 1/5 more armour to retain absorbtion.
If you go from level 50 to level 51 you need 1/50 more armour to retain absorbtion. I think this is quite easy to remember.