Whaaaaaaat? HQ gear, materia melds, and food; simple as that. You can go way waaaaaaaay over 315 (for example,
my weaver is sitting on 350 control without any food), so I'm not sure how you could possibly not see how you could increase it. Salt Cod Puffs would give you +18 control (and they're not even the best control food, just probably the highest reasonably priced one), so you're not even that far off.
You're a blacksmith, so my first suggestion would be to bang out your own HQ Militia File. That would add 6 control, to put you at 286. Salt Cod Puffs puts you at 304. A mere 4 tier-3 control materia (because you probably don't want to shell out the cash for tier-4s) later and you've breached 315. And that's not even your only way to hit 315.