Agreed, they photos must be hard to compose, and I know that they're hard to expose. This is one of those situations that auto-exposure won't work very well. Since you have those long petals, it means your photo has LOTS of that black background in it. If you want the exposure to be perfect, you'll have to set your exposure compensation at about -1 or 1.5 f stops.
With your D30, you can do that using the BIG dial on the back of your camera... and work it with your thumb. When you're looking throught the viewfinder and it shows the exposure, you can turn that dial (if it's on) and it will slide a little pointer to the left or right. Just remember to set it back to the middle when you're done.