Hello,
I see that the amount of targets for concecrate changes the amount of spell damage gems that the sim suggests. What is the recommended number to put in use. For normal/heroic tanking at least. maybe through kara, grull and mag.
In general, there are multiple targets only for specific portions of fights. You should decide which part of the fight you want to optimize for. If you care more about the parts with multiple enemies, try to guess on average how many you will hit at a time and set it there.