People say clip Thundercloud at 12s, but if you used a Thundercloud before, wouldn't it be already possible to clip at 15s?
24s -15s / 3 = 3-4 ticks. Thundercloud deals 390 Potency + 40 Potency/tick, at 3 ticks it's a total of 510 Potency for 1 GCD + movement, as opposed to 504 Potency (280*1.8) from F4 with a 3s cast.