It'd be a lot easier if the game had something to simulate accuracy, and if needles scaled that factor simultaneously with potency. At that point, against a single target, you on average hit with the appropriate number of needles, facing bell curve fall-off to any greater or lesser amount. But against multiple, especially if surrounded on all sides, you've got some real firepower available to you.
In the meantime, though, it'd be far easier just to give it two maxima.
1000 potency overall maximum.
300 potency max per enemy. (Or whatever you think is appropriate for its CD.)
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(More broadly...)
My real issue isn't that having a traditional name attached to it restricts fine tuning--that's easy enough to get around--it's that the identity feels a bit muddled. On a third and fourth read-through that's actually continually the impression I'm getting from all this. It's part Squall, part Soldier, part Blue Mage, and part Rune Knight, yet it doesn't seem to cohesively hold together those parts as well as it might, even while ensuring that there are now three fewer future jobs available due to each having some small part of itself embedded in this other job.
Blue Mage felt eccentric and eclectic. It looked wild, following a very unrefined sense of pragmatism. But that's about as far as you can get from Garlean methodology. They're precise, essential, and modular, with a penchant for over-development and exclusion in the pursuit of order or streamlining. Every magitek ability the Garleans have used against us has been in the shape of raw power, basic but large cure, fire, blizzard, lightning, shadow, freeze, flare (shadow or fire varieties) spells, and in one case, stoneskin when that was the only means of shielding. No magic missiles, no needles, no aqualung, no white wind or arctic wind, nothing of special effect: just sheer force. Squall's Rough Divide, Magiteck Overdrive, Blasting Zone, Degenerator, Aetheric Overload, and Beta Wave all seem appropriate largely for that reason. But the goofily "BLUish" spells, that made BLU be BLU, don't seem like they'd fit in a suit of Garlean techplate.