Here’s another pretty useful community tool. If you’ve been dying a lot of just getting a ton of damage thrown in your face, this one could help!
SkyLineOW calculated the armor breakpoints vs. monster levels, finding the amounts needed for maximum damage reduction (85%). The actual max you can ever need for a level 154 mob in NMD 100 comes in at 13441!
Source: I (mostly) cracked the armor formula and made a ~98% accurate armor calculator.
You can just create your own spreadsheet by going to File > Make a copy. But, all the relevant monster levels are right there in the base one anyway so you don’t really have to. Unless you really want to know how much you need under level 50, but in that case you probably have other problems with your build!