One Simple Rule of Proportion: Two-thirds
It is a simple rule of proportion, merely let your eye divide one-third, two thirds to keep things a bit more interesting.
Take your living room; the furniture should take up two thirds of the area, leaving enough floor space for easy traffic flow. The painting you hang behind the couch, table or above the fireplace should be approximately two-thirds the width. Your coffee table should be two-thirds the width of your couch. The accessory on the coffee table – the flower arrangement – should be two-thirds the height of the vase.
If you’re shooting snapshots or designing artwork,two-thirds will have more appeal.
When symmetry becomes the focal point, static and mundane will bore you. For example, the dark trees and horizon lines, on the left, follow the two thirds rule, whereas the cropped versions on the right is divided in half. Which do you find more interesting?
For you math majors and my engineer son, the Golden Means Triangle and the Nine Grid Zone (think tic-tac-toe board) offer an elaboration on this two-thirds theme. Even, I, an artsy shopkeeper, use these formulas. They aren’t difficult to learn, they just require a bit more time and calculations. Maybe you'll find them helpful.