Nature's Grace plays a very important role in the solar rotation (and in the lunar rotation). Among other things, it creates a soft cap of haste rating for druids beyond which haste becomes less effective on a per point basis. Keep in mind this is a soft cap, not at all like the cap on hit, after which a player receives zero benefit from additional points.

My motivation here is that I am trying to build a spreadsheet to calculate my theoretical DPS. One of the most important components in this calculation is cast time. The haste part was easy, but after hours of scouring the internet, I was unable to come up with a reasonable formula for finding the uptime of Nature's Grace. (apparently I didn't look very hard) Woe is the balance druid's underpopulation. And so it was that I began re-inventing the wheel to make my own spreadsheet. There is was a lot of math here. If you're not interested in that, hit ctrl+f and search for TL:DR. Your brain (and eyes) might thank you.

Allow me to steal a page from Graylo and talk about my theoretical raid buffed druid, casting with 48.8% crit, 3061 spell power, and 22.2% haste. I am ignoring on use trinkets and eclipse bonuses. I expect the mob to have moonfire on it while casting starfire.

Allow me to quote Murmurs:

Nature's Grace: Buff or Nerf or what?

This one is pretty much a no-brainer. For Balance PvE DPS, this is a huge buff, probably one of the largest buffs we've seen since they re-adjusted the base-line damage of Starfire and Wrath to non-suck levels.

Here's how the new Nature's Grace formula works for calculating cast time:

((Base Cast Time / (Haste Effects)) / ((1 + (Haste / 100))) / (1.2)

Average cast time formula should look like this: ((Base Cast Time / (Haste Effects)) / ((1 + (Haste / 100))) / ( 1 + (.2 ((1-(1- Critical Strike Chance)^Number of Casts)))

Using the numbers above, Starfire's average cast time should look something like this:

((3 / 1.114) / ((1 + (18.15 / 100))) / (1 + (.2 * (1-(1-.3749)^2)) = 2.03

Under the current model Starfire's average base cast time should be this (note, I'm auto deducting the effects of Celestial Focus, Improved Moonkin Aura, and Wrath of Air Totem from the base cast for simplicity)

(2.7 - (.5 * .4049)) / (1.1815) = 2.11

While the proc rate of Nature's Grace hasn't changed, it still has the same effect on Starfire's cast time and now efffects 2 spells instead of only one which is why the average cast time is going to be lower than the current version. The only time there would be no net change over the Live version and new version, for Starfire, is during 100% critical strike situations - so basically Loatheb.

And now allow me to sum that up. See that little part that I put in bold? That's the part that tells us how to calculate the uptime. Think of it like this: We assume that Natures Grace has just procced. Then the chance of that buff staying up is equal to the chance of one of the three wraths critting. This is the uptime. So in go my numbers and out comes my uptime! Here is the theoretical chance that one out of n casts is a critical.

n Casts	Wrath	Starfire
1	48.82	51.82
2	73.80	76.78
3	86.59	88.81
4	93.14	94.61
5	96.49	97.40

My theoretical druid has an 86.59% uptime on Wrath and a 76.78% uptime on Starfire. Notice that this coincides with the percentage chance of at least one crit per the number of casts in one Nature's Grace proc. Back to the spreadsheet to continue plugging in numbers.

TL:DR
Calculate your Nature's Grace uptime by using (1-(1- Critical Strike Chance)^Number of Casts) for each spell. Remember, Starfire gains crit from both Moonfire and a Lunar Eclipse. Take that into consideration as you are calculating.