Shooting a macro of a flower at close quarters with long focal lengths (say 200mm) can present a problem with obtaining sufficient depth of field.
You’re basically trying to maximize the DOF in order to get as much of the flower in focus as possible and to avoid having only the front portion of the flower in sharp focus and a blurred rear, or worse a sharp rear portion and blurred front.
2 images illustrate the problem: Image 1 | Image 2
Stopping down the aperture of the lens to f/11, f/16 and beyond increases DOF, but presents the issue of increased of diffraction effects that result in images with decreasing sharpness (although Peter iNova asks, is this necessarily bad? and writes a nice post on it)
Getting back to the question of getting as much of the flower in focus as possible, you’d need to observe the 1/3 rule — the point of focus should be one-third of the way into the area of the subject you want in sharp focus » read Gabby498’s excellent explanation on the technique.
Related
Photography Tips — Main page.