You can queue before the end of cast safely, and most custom castbar addons will happily show you the safe zone based on your current latency, etc.

I am looking for a way to be able to queue new spell casts within a reasonable timeframe of the current cast (0.2-0.4 seconds?) without having to rely on setting Custom Lag Tolerance high as that just raises new issues.Your description of the situation without a custom lag tolerance set is. I am looking for a way to be able to queue new spell casts within a reasonable timeframe of the current cast (0.2-0.4 seconds?) without having to rely on setting Custom Lag Tolerance high as that just raises new issues. At low tolerance there is a gap between spells, and at high tolerance I can't back out of a cast if new information arises that would make me change my mind. I have also tried manually adjusting the lag tolerance, however I just run into the same problems. For example if I am casting Frostbolt then decide to queue an Ice Lance, only to change my mind and cast Frostfire Bolt due to a proc, it is too late and Ice Lance goes off anyway. If my Custom Lag Tolerance is set high, then the window in which I can queue up a new spell is considerably increased, however this isn't ideal since it seems to lock in the choice quite early. I have tried using Custom Lag Tolerance, with an addon that adjusts the tolerance to my current ping, which is important because as an Oceanic player my ping regularly varies between 50-250 depending on whether I am on an Oceanic server or in a dungeon/pug on an American realm. There seems to be a ~0.1 second window where I can queue a spell while still casting the previous spell, but I don't want to get an early onset of arthritis by trying to mash a key 10 times per second. For example, if I am spamming Frostbolt I can't queue up the next Frostbolt cast until the previous one is finished, meaning there is continually ~0.1 second gaps where I do nothing, which obviously add up over the course of a fight. I have been having issues with queuing up spells to be cast the moment the previous cast finishes. TLDR: I want a way to queue spell casts without using high Custom Lag Tolerance.

Wow set spell queue window