Tank simulation is a bit different than DPS simulation, there are a lot more variables. It is very difficult to simulate tanking exactly as it will happen in the game. Therefore you want to interpret the simulation results on a “relative” scale rather than an “absolute” scale.
The actual M+ level that you choose in the simulation probably won’t match up exactly to what you can do in-game. This is why we call it “NPC Damage Multiplier”, with entries like “+85 % (Mythic+ 10)” – the “Mythic+ 10” part is just a suggestion to help you pick a starting value. To get good simulation results, you might need to move that up or down until your character is at a reasonable death chance.
If your death chance is too low (under 5-10%) or too high (over 60-70% or so) then the results of the simulation won’t be very useful. The reason for this is pretty intuitive: let’s say you run a simulation and it shows that you have a 100% death chance – there is simply no way for you to survive the fight. You check the log generated by the simulation (with “Report Type” set to Log), and you are dying within seconds of the fight starting. Now let’s say you add 20 stamina to your character and simulate again. You’re still going to die horribly… there’s no way for the simulator to give much useful information because for all practical purposes your toughness has not improved.
The same thing happens if you are too tough – say you simulate and have a 0% death chance. You look at the log and you barely need any healing to stay alive either. Now let’s say you add 20 stamina to your character and simulate again. Well… you’re still invincible. It’s hard to give any value to that stamina because you’re already too tough.
So tweak the NPC Damage Mulitplier until you have a death chance around 20-50%. This means that you get into trouble sometimes, but not all the time. In this range, the simulator is very good at estimating the relative value of adding different kinds of defensive stats and effects to your character.
Though this doesn’t match up exactly to the absolute amounts of damage you are taking in-game, and the simulated healers don’t exactly mirror how your teammate is healing you, the overall gist and patterns are similar enough that the relative value of defensive effects can be estimated very well.