Anyway, biggest thing I found with a hunter - don't think of Cobra and Arcane shot as belonging to a "priority order" like a caster for instance. Don't think that you should "cast this with X frequency" or "cast it with Y frequency" - you don't cast 1 Arcane Shot every 3 Cobra Shots, or some crap like that. How frequently you use arcane shot isn't something you control preemptively, it's something you react to based on your situation. More haste? You'll arcane shot more. More lucky procs? You'll arcane shot more.
You're a resource class, think more like an Energy based class. You regenerate Focus with Cobra Shot (akin to "pooling energy"), and you use Focus on abilities.
Your goal is to have enough focus for your abilities every time they come off cooldown, but not to cap it, or you are wasting your focus regen + cobra shots. So you use Arcane Shot enough so that you never cap, but not so much that you won't have it for KC. Resource management 101.