Theoretically the exact point where the red line starts would be your start casting time, anytime you hit beyond that is time between casts. If trying to maximize dps you are either going to clip the last missle or have dead milliseconds between casts
just due to human error or reaction time. It gets much harder when all 5 missiles are going off in 1.5-1.8 seconds, and you are trying to watch other things on your screen besides cast bar like big wigs bars, threat meters, and proximity meters.
In my case this is also in addition to raidleading. I rarely didnt clip missiles when under cds, and know that im losing cast time if im not clipping.