"Recipe"-based Magic System - how to resolve/refine?

[Geethree]

I feel like I need a system that simulates the "recipe"-like nature of magic. In other words, magicians should be able to put together a new spell on the spot and the system should support that. In JS&MN (which, again, is an inspiration but I am not trying to copy it directly), I feel like a lot of the magic was like that. Characters took a lot of "remedial" spells and then worked their way up, casting more powerful or variant versions of those spells. I like the idea that the players have control over their magic and aren't just learning more powerful spells from the Spells chapter in the book. If you look at most of the spells in JS&MN, I feel like you can see how they would be built using the framework of this sytem.

In addiition, one thing I liked about JS&MN was that spells seemed to more or less work. Once magicians learn a spell, it happens. When Jonathan Strange animates dead soldiers and speaks to them, he doesn't say the magic words and then roll a die.

Yes, magic should still be a little unpredictable, and I eventually want my system to reflect that (there are several things I want this system to eventually do too but they're not as important right now), but I want to build off the a+b=c foundation because it gives a lot of freedom and creative control to the players. If you doubt whether it suits the tone of my game, all I can say is that I think it does and I hope that you can trust me on that because I'm not really prepared to type up a lot of huge posts about my setting

Right now my difficulty is getting these mechanics towards something playtest-worthy.

[Xeven]

This sounds pretty cool.  I was playing around with this idea a bit and came up with a kinda chart and formula for casting spells though it does kinda make things more complicated. Maybe it will help you out somehow anyway. Oh I clumped elements in with nature for this example and just dumped in random points.

                                  Potency            Range (Feet)                Mass (1cubic ft.)           Duration (Seconds)         
Command                         1                    6                                    1                                 2
Conjure                             1                   6                                     1                                  2
Imbue
Sense
Commune
Life
Death
Nature                            2                     3                                    1                                  2                     


So say you wanted to cast a fireball and then hurl it at somebody...
It would be...

(Conjure + Nature(Fire) +Command) with the formula being...

Conjure
Potency-----1 (You are able to conjure 1 thing.)
Range-------6  (You can bring anything you conjure into existence within 6 feet of you.)
Mass--------1 (The mass of anything you conjure can be up to 1 cubic foot.)
Duration----2 (Anything you conjure lasts for 2 seconds after being conjured.)

Nature
Potency------2 (A nature spell you cast may deal or prevent 3 damage.)
Range--------3 (A nature spell you cast may have a range of 3 feet.)
Mass---------1 (A nature spell you cast may be as large as 1 cubic foot.)
Duration-----2 (A nature spell you cast ends after 2 seconds.)

Command
Potency-----1 (You may command up to 1 thing.)
Range-------6 (If anything you command moves more than 6 feet away from you, you lose control of it.)
Mass-------1 (You may only command up to 1 cubic foot of anything.)
Duration----2 (After 2 seconds you lose control of anything you command.)

...and the result being...

You summon one, 1 cubic foot fireball within 6 feet of you that you project 3 feet and it does 3 damage and disappears within 2 seconds.

Yeah a tad complicated but maybe it will spark a new idea for you. My thought was to possibly have a limit to the amount of spells players had knowledge of so they could calculate the formulas ahead of time which would still allow them plenty of creativity. But that may be stearing away from what you were originally going for.
Well anyways good luck with your game I hope it turns out good.