Of course, in the series finale for TNG, Riker gets to say “warp 13” or somesuch, which blows the TNG formula away.
However, like any good grasping-at-straws Trekhead, I have come up with a tidy rationalization. 
It seems clear that in that future timeline shown in All Good Things, ludicrous warp velocities can be achieved. In other words, while in ordinary TNG, Georgi starts to sweat when Picard pushes the ship past, say, 9.6, and the bolts start to vibrate at 9.7, we can assume that in the future, 9.9 is not a problem. And since speed increases logarithmically and asymptotically, we can approach warp 10 but never achieve it. However, assuming radical improvements in technological capability, it quickly becomes unwieldy to say, “Helm, warp nine point nine nine two,” or whatever.
So a new terminology is invented, in which each number from warp 10 upward is understood as a shorthand means of adding a decimal nine. Warp 10 = 9.9, warp 11 = 9.99, warp 12 = 9.999, and so on. Warp X point Y works the same; 10.5 = 9.95, 13.4 = 9.9994, and so on. So Captain Riker, in the spiffy new Pentium Google warp ship, doesn’t have to say, “Helm, warp nine point nine nine nine nine nine nine two.” He can simply say, “Warp fifteen point two,” and the Academy-trained helmsman (or the computer, more likely) makes the adjustment.
However, given that the formula “n=c*n^3.333…” specifically says “for values of n up to 9,” it’s hard to say exactly what those higher warp numbers actually represent. If you use just that formula, you find diminishing returns the closer you get to 10; the difference between warp 13 and warp 14 is a measley six-tenths of c, which hardly seems worth the effort. But the graph in the reference book shows a huge spike after 9, so Lord only knows what warp 9.999 might actually be. It’s possible that 9.999 represents some totally implausible value by which it’s reasonable to get from one galaxy to another.
Even so, I thought it was a nifty way to explain something that seems to violate the “rules” and which I’ve never seen accounted for anywhere else. Thoughts?