Okay, here's a an answer because I've explained it a hundred times and I'm getting annoyed:
6% to Nature. 6% to Arcane. 12.3% to Astral damage (this is confirmed).
It takes 9sec to fully stack at 33% spell Haste. (1% every 1.5sec | 2sec / (1+HastePct)).
Over 9sec, that average bonus is simply 3% nature/arcane damage or 6% for Astral damage.
An encounter may last anywhere from 4min-15min, call it 8min because that's normal. 8min is 480sec. That's 471sec of uptime on the maxed debuffs (98%). The remaining 9sec were 3%. Assuming 33% Haste, this means we have a maximum of 320 or 319 ticks (640 or 639 ticks, but it doesn't matter). 320 ticks * 6 = 1920. We know we lose 6 ticks due to ramp up time, but they are worth 3% on average (3). 6 * 3 = 18. 1920 - 18 = 1902.
So the effective debuff is 1902/1920 = 99% of 6% debuff.
This translates to a 6% debuff in single-target (shocking!). So we can just assume 12.3% for Astral damage because it's effective debuff is 99% of 1.06 * 1.06
This means CI is effectively a 6% increase outright, not including the bonus to Astral damage.
According to logs, Astral damage deals about 30-40% our of total damage in single-target. 30% for most players, 40% for those with ED.
Code:
12.3 * 0.3 = 3.69 / 2 = 1.845
12.3 * 0.4 = 4.92 / 2 = 2.46
6+1.845 = 7.845
6+2.46 = 8.46
7.85 - 8.5% effective buff in single-target. This is fairly consistent with my 8-9% claim, albeit the top-end was a tad high.
If you want to figure it out for AOE, then it's someone else's turn to step up.