The Golden Ratio - 1.618
The Golden Ratio is a number which can be used to create harmonious layouts and naturally pleasing designs.
The ratio itself is called Phi or 1.618. It is derived when two quantities are divided in a way wherein the ratio produced is the same as the ratio of their sum to the larger one of the two quantities.
Excerpt from Wikipedia
Here is an awesome video that ties everything together!
In conclusion, the Golden Sprial can be used to provide a framework for all your design decisions. You will need to practice before you can start to implement these ratios into your logos. The more you use it, the more you get accustomed to referencing it when creating various design elements.