First however you want to handle it in your game is fine. To help get stricter spells, just make the names less open to interpretaion, more prosiac and less poetic when y9u design your own. I can definitely see an argument for a spell always does the same thing - though I might wait until first attempted use before pinning it down, YMMV. If you can handle the bookeeping, record the usage and then always make sure you use that as the effect after that.
If you want more flexibility and are looking for a justification then something along the lines of 'Professor Mhurzam was a brilliant man, but even he should have realized that by varying the quantity of bat guano the explosive force could be re-directed as a propelling power. Of course this variation is not without its dangers..'. Or to see it another way it could be possible to 'hack' spells like computer programs to vary their effect, but at the risk of reducing ther reliability, predictability and safety (i.e. give them a penalty). Some cultures (Brithini) might frown on spell 'hacking' - "For Malkion's sake, you know perfectly well no spell variation shall be completed without a full functional requirement and test plan being submitted to the procurator..."
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