Close to is any point when your not just over that rating as presumably their the points you gain another tick of immolate, so when your not quite there yet the duration of immolate is shorter making it harder to hit with HOG.
Working on that math now. Initial estimates put it at 200-300 haste rating below each of those caps.
---------- Post added 2011-01-10 at 06:15 PM ----------
Basically, at 300 under each cap, you will have a rough .6s window during which to refresh. As you approach the numbers specified, it will drop as low as .3s. In my books, .5s is too small a window because of pushback mechanics, so you want to stay out of this range. However, i'm unsure if this will cause your dps to be terrible as simcraft plays much better than I.
This is assuming no travel time, which I need to test.