This custom Weak Aura provides an accurate indication of when you are able to queue your next cast/ability regardless of whether it has a cast time or triggers the GCD. This reduces the need to pre-spam your next ability. (I still recommend a double-tap, single doesn't seem to register sometimes)
It utilises the following functions: GetSpellCooldown(61304), GetCVar("SpellQueueWindow"), UnitCastingInfo("player"), GetTime() to ensure accuracy regardless of the setting of the spell queue window value, what spell is being cast, or the speed of the global cooldown.
I highly recommend converting/adjusting the aura to your preferred texture/icon.
https://gfycat.com/ImmediatePlushBarnacle
You can set the spell queue window (max 400) with /console SpellQueueWindow X
You can check your current spell queue window with /dump GetCVar("SpellQueueWindow")
updates:
v2 - included progress and duration data, can now be used as a progress bar/texture!
v3 - now a group with an icon and progress texture (cooldown style) also smaller, more practical