The original Magic Formula, written by Jim Henley, can be found here. It is a wonderfully elegant codification of magic that makes it flow much more smoothly for Everway in any sort of long-term play. Nonetheless, I felt there was room for both improvement and expansion. Below is my attempt to complete it. If you're satisfied with Everway's magic rules already, this is probably not for you.
Aptitude + Performance [+ Bonus] = [Penalty +] Target + Duration + Effect
The Magic Formula and Character Creation
Using the Magic Formula, school design changes slightly. Since the degree of effect of a particular action (spell) is determined by the Magic Formula, the school writeup should focus on two things. First, how one performs the magic. Second, what new things can be done at each level. Every school of Magic should have at least a half-dozen distinct abilities available as the magic level increases.
With the Magic Formula in place, a character's Magic score may now rise higher than its associated Element. Each point of Magic increases the variety of spells a mage can cast - and a character's Element limits the maximum Effect possible without outside effort. Thus they already balance against each other, and there is no need to artificially limit one.
Note that there is nothing stopping a magic school from being based off a hemisphere instead of a single Element. Also, there should be no reason why a character cannot know multiple schools of magic, apart from a limit of points. However, neither of these ideas have been tested in play.
The Magic Formula in Play
The two values that make up the character's Aptitude - Magic and Element - only rarely change, and then at monumental points of an Everway campaign. So a player just needs to add them once and remember the value. The Casting Time, then, is up to the caster, within reason - it's hard to chant and dance when you're bound and gagged, and even if nobody's stopping you, eventually fatigue sets in.
The values on the right side of the equation interact in more complex ways. Generally, the magician first decides what his or her aim is - in other words, the target. From then, the magician decides which is the more important - Duration or Effect, and sets that value. The third is then calculated. Note that the Effect cap is also pretty much a fixed number, so it should be easy to remember.
Note that small increases in Performance can net huge increases in Duration, Target or Effect. All else being equal, Performing for a minute instead of casting it instantly, he can change the duration of a spell from a day to a week, or a weak effect into an average one. Ritual is good, if you can get away with it.
The Magic Formula and the Elements
The element governing your magic school figures directly into the formula. But there are other uses for elements.
Your Earth score determines how long you can Perform a spell without keeling over completely. Generally speaking, your magician can spend units of time equal to her Earth score in ritual vigil - this means a mage with 3 Earth can spend an entire day performing. An Earth 5 magician can spend a month invoking a single spell (note this doesn't mean that they don't stop for eating and breaks, just that they do nothing else during that time).
The number of "instant" spells possible for an Earth score remains undefined. GMs may wish to rule that certain schools are limited in the lengths of rituals possible - or, perhaps, certain schools have minimum casting lengths.
Other elements come into play as necessary - an illusion, for instance, might have to match its Effect against a character's Water score.
The Magic Formula and Overcasting
Occasionally, magician might wish to overstep his bounds, attempt to cast spells for which he is not yet ready. This is most often the case amongst the smallest of apprentices, which is why Everway describes those with a Magic score of 1 "a beginner, capable both of modest tricks and catastrophic mistakes." The magic formula up until this point easily takes into account "modest tricks," but seems woefully unequipped to handle "catastrophic mistakes." This is where overcasting comes in.
A mage overcasts whenever he attempts to access capabilities provided by a higher level of Magic than his Magic score. This imposes a Penalty equal to the difference between the mage's Magic score, and the Magic score required for the effect. So if an Apprentice (Magic 1) tried to cast an advanced spell (Magic 4) the Penalty value would be 3.
Overcasting also requires the character to draw a number of cards from the Fortune deck equal to their current Magic score (regardless of the level of Magic being attempted). All cards drawn must be beneficial - otherwise, something has gone wrong. If only one card goes wrong, perhaps the spell just fizzled out. If they are all bad cards, the spell has failed catastrophically. No matter what, all the drawn cards influence the outcome.
Yes, this means that an apprentice is more likely to overcast successfully than a skilled mage. That's on purpose. But the apprentice has a 50% chance of failing catastrophically, whereas the more skilled mage is drawing additional cards that will help ameliorate his failures. And that apprentice is going to be taking huge Penalties for any high-level spells.
For those who want overcasting to be more dangerous for the unskilled, and otherwise more muted in effect, use this rule instead. When overcasting, the magician draws a number of cards equal to the magic level being cast. If all the cards are successful, it works correctly. If even one card fails, the spell fizzles out. If the number of failed cards is equal to, or greater than, the Magic level of the caster, the spell failure is catastrophic. This allows callow apprentices to have catastrophic failures (one card failures) while severely reducing the chances of successfully overcasting. If using this optional rule, do NOT impose a Penalty on the magic formula - successful overcasting is much rarer, negating the need for one.
The Magic Formula, Bonuses, and Penalties
Bonuses and Penalties vary widely with the circumstances of casting, as well as the school of Magic being cast. Possible bonuses include:
Penalties may also result from any number of circumstances. Calendar effects, the range between magician and target, environmental conditions, unholy or unmagical places. And, of course, overcasting.
The Magic Formula Off The Scale
So how do you cast a spell that lasts longer than an Age? How do you cast a curse on more than one sphere at a time? In short, how do you get a supernatural effect "off the scale."
There are two approaches. One is to make such effects entirely at the whim of the GM. The other is to rule that one can only get off the scale with some kind of Bonus, most commonly a sacrifice or calendar effect. Now you know why those priests are tossing everyone into the volcano during the perigee of Halley's Comet.