I generally try to wait until I get a Blunderbuss Proc to use Between the eyes unless you're using Quick Draw which might be better at low gear levels since it's more free CP = more energy free for the rest of your rotation in which case it is likely worth it to use on Cooldown with your Pistol shot procs and blunderbuss procs. You can take a look at my warcraftlogs parses in my signature if you're curious what sort of DPS I'm doing without Frond(About 1.2-1.35 mil dps)
https://www.warcraftlogs.com/reports/dcHG1hXT8g42Jf7V/