Missile Barrage proc rate

I haven't been arcane since really low gear, I've respecced back to arcane yesterday but I've noticed that I reach 4 stacks of the debuff without it proccing at 5/5 (40% proc chance). My question is should I pop one point into barrage to use Barrage if I have 4 stacks and no proc or just spam some more blasts to get the proc? At 4 stacks it costs a ton of mana, not a big issue for shorter fights, but it's annoying af to spam 3-4 blasts more at 4 stacks.

The rotation shouldn't change. If the fight is long enough for mana to be a concern, you'll want to be mindful of the damage per mana of your spells, which means you'll just Arcane Missiles at 4 stacks to reset stacks even if Missile Barrage hasn't procced.

The previously bugged proc rates caused lots of people to expect a MB proc to accompany every single AM use. That's not how the spec is meant to be played.