So I've been playing TES since the release of Morrowind (and have even gone back and put some time into Daggerfall and Arena). And I never really use spells like Calm, Fear, etc... But since with each new game Bethesda streamlines the games more and more and we have less and less to play around with, it's getting to the point where I should give them a whirl. The issue is, I've never really understood how they work and have had trouble finding a decent explanation of it. See, the magnitude determines the "Up to Level" parameter... but what the [censored] does that mean? I have never in all my time playing been able to look up the level of an enemy, so how am I supposed to know when something will reliably work on them? Idk why there's no description of what that means because it's a very poor/vague explanation. If a calm spell only works up to level 2, does that mean after the first 10 minutes of playing the game that spell is now useless? Someone explain this.

You can find most of the levels of enemies on the UESP.
If you play on PC just get their level with the console.

You have to use your noggin and guess which enemies the spells will affect. Take your own level into account as well as the type of enemy. Are you level 10, using a level 10 Fear spell on a regular bandit? Then it will probably work! Are you level 30, trying to use a level 10 Turn Undead against a Draugr Deathlord? Yeah, probably not.

Edit: Or, as stated above, look up the levels, but where's the fun in that? = D

I'm guess let me say that again "I'm Guess " not sure but I think your enemy's level up as you level up so you should use spells that about your level

So the spell's magnitude literally depends on the numerical level of the enemy you're fighting? That's such piss poor design since that's not listed anywhere within the game or documentation. They would have been better off with the same type of deal as Reanimate, using a generic descriptive word so as not to lead you to thinking you're supposed to know what the [censored] that number is for.

If the spell says level 50, btw, it will affect every enemy in the game I believe, I dunno if they go that high though.

They do it depends we're you put your perks like if you shock is maxed out you can do I think a 60 or 90 level thunderbolt damage spell

I usually just guess. I'm only wrong once in a while.

pretty easy to determine by the name of the enemy. lower level spells will only work on standard bandits/creatures for the most part

I love the idea of the illusion skill, but found it poorly implemented in skyrim. You basically HAVE to take almost the full perk tree in order to keep it leveled up with you.

As you level up get the next level spell. Always use the highest level you have. If it still fails, double cast. Double casting increases a spells strength. If you are having issues running out of Magicka with the higher level spells just get Magicka use reduction gear for that particular school of magic. It's not that difficult and really doesn't involve guesswork or a real need to know enemy levels.

By the time you can get the next spell level your enemies are probably already scaled to a high enough level that the previous spell will not work anymore. I mean, unless you're trying to calm a deer I wouldn't really be too concerned with using a spell level that is too high. Just use the best one you have and if it takes too much magicka for practical use just go down to the next spell.