As a Marksman Hunter who uses Aimed Shot as a focus dump instead of Arcane Shot, I tend to view my Focus bar differently from how I used to view it as Survival, Beastmaster or even when it used to be a Mana bar.
I see two things: block reserved for Chimera Shot and block reserved for Aimed Shot.
Each one of those takes close to half the focus bar, so I generally view the bar in halves. Usually the first half is viewed as Chimera Shot and the second half is viewed as Aimed Shot but this can shift based on what is available for consumption, how close to coming of cooldown Chimera Shot is and where my haste is at for the moment. I think of it as shuffling these blocks around.
I quickly process whether I can squeeze an Aimed Shot off and still have enough Focus for Chimera Shot or enough time on the cooldown to get enough focus back (usually two casts of Steady Shot) for the Chimera Shot. In the case where I have enough focus for an Aimed Shot but Chimera Shot is coming off cooldown in about three seconds, I’ll use Steady Shot a couple of times. If I’m in the middle of a third cast of Steady Shot, I will interrupt it and cast the Chimera Shot and follow up with Aimed Shot immediately.
I find this helps me quickly decide whether I should cast Aimed Shot or Steady Shots and with enough practice, it doesn’t require as much processing.
I almost never use Arcane Shot unless I’m on the move (in those cases I sometimes switch to Aspect of the Fox and fire some Steady Shots if I don’t have a full Focus bar) or when I need some quick damage (like knocking out totems in the Troll Heroics).
This is what works for me but run with what you’re comfortable with.