Honestly, I've never really had this issue come up in any of my games. The reason why, I think, is that I don't approach it from a "building encounters" perspective. I never sit down and say to myself, "Okay, I have 4 players, and if I want this fight to be pretty challenging then I'll need to make the bad guys this strong or that numerous."
Instead, I approach things from a story perspective- one of the things that the MAG is VERY well designed to do. My villains all tend to be personally involved with and of great significance to the players. I often find myself asking myself things like, "Okay, if I was this villain, how would I respond to the heroes' actions? Well I'd probably set an ambush here. How many lackeys would the villain feel like is enough to bring and overwhelm the heroes?" And etc.
The result is that I don't worry too much about crafting specific encounters to challenge the heroes just so much, and no more. Instead, I set up situations and scenarios and throw the heroes in the middle of them, then let them figure out how to handle them. Sometimes they slaughter the opponents, sometimes they get their trash kicked and wind up imprisoned. The point is, no matter what happens, I try to set it up so that it advances the story in a significant and fun way.
Now, just in case that doesn't satisfy your question, I will say that I tend to err on the side of lenience towards my players, at least when it comes to fighting extras (I like my villains to be a challenge). Typically players will have more fun mowing down a half dozen house guards than they will getting their trash kicked by two dozen hazekillers (of course, there are exceptions
Another thing to always keep in mind is to have something for everyone to do, but especially if you want your fight scenes to be challenging. If you throw one super-powerful villain at your players, the villain is going to go down, and fast- especially if there are any Feruchemists amongst your players. (again, exceptions
). Much better to throw multiple villains at your characters, or at least one villain with a bunch of extras serving him. Or set up a scenario where the heroes have to deal with multiple challenges at once. Something like some of the heroes have to break the prisoners free before the executions can begin, while others of the heroes try to stir the crowds into a riot, and still others have to dispatch the guards on the canal boat so that the prisoners can hop aboard and escape, and then in the middle of everything, that Inquisitor with the distinctive scar that has been hunting Hero X shows up again grinning wickedly. Something climatic and exciting like that. It doesn't matter how hard/easy the fights end up being, or what the outcome is, you and your players are sure to have an enjoyable experience.