Armor is only a part of your defenses. It’s effectiveness is also tied with your health, so you should work on a balance of both to get the most out of each.
It also depends if you’re in softcore or hardcore; obviously in HC leagues you’ll want to get higher defenses before you tackle 72+ maps. Also important to note are levels, being the same level as monsters or 10 levels above can make a huge impact to your overall survivability. For level 72 maps I’d recommend you try to be level 82 at least.
You also didn’t mention if you have Iron Reflexes (turns evasion into armor) or not? Obviously if you have 50% or higher chance to evade attacks then you don’t necessarily need as much armor.
Since armor has diminishing returns (same amount of armor gives you less physical reduction with every level), I’ll just mention roughly what amount of reduction you should aim for.
With Iron Reflexes, I’d recommend you try and keep your physical resistance over 40% at all times. 50% or more would be fairly safe but 40% I think is a minimum you should have, and it’s not hard to achieve. Without IR, try to stay above 20% physical reduction. As mentioned health plays an important role here as well, and as such having 2500 health and 50% physical damage reduced is fairly bad, but having 3500 health with 40% reduced phys damage is much better (from my observations).
These are all just rough estimates from the top of my head and I didn’t do any effectiveness calculations or anything like that, so you don’t have to blindly follow it. If you’re always playing in groups less armor and health can work well since damage taken is spread to multiple targets.
It’s a bit outdated, but this defenses analysis research by Lord Krike can provide you with further insight into defense calculations, effectiveness, mitigation and all that stuff.