So are you saying the ideal ratio for stamina and armor should always be the same? Or that you are looking for a ratio of 1:20 (I'm assuming that would mean 1 stamina to 20 armor).
So, what should you aim for when choosing gear? What we see here is that balance is pretty key to the whole thing. If you stack one at the expense of the other, you are doing yourself more harm than good overall. Having a good balance between armour and stamina tends to focus both for you - when you are taking physical damage. Back in Burning Crusade, the 12000A/12000H pair was thrown around as a "good starting point" for Karazhan raiding. We see that this pair tends to be around 1:20. For Wrath of the Lich King, it seems that 20000A/20000H will be the goal.