What kind of bike are you riding?
The way I see it, the chainrings (the two or three big rings in front) aren't there for fine-tuning. They each have their own uses. Big ring is for going fast on flat ground (once your legs get there!) and descents, middle ring is all-purpose, and the small one is for climbing hills. You need to fine-tune using the gears in the back. Now, you'll probably feel like you're spinning too fast for the gear. Unless you feel a little jump at the top of each pedal stroke, you probably aren't.

I will say, though, that 8-speed cassettes seem to have climbing gears and "zoom" gears, but not cruising gears.