There are no set rules as to when you HAVE TO transfer.
Most of our opinions are based upon our experiences.
The 1-2-3 method works OK most of the time, exept for high gravity brews.
Many people rack when the gravity has dropped about 80% from the original (too restricting) while some rack when they feel like it.
The main purpose for racking to the secondary is to get the brew off of the dying yeast so you don't get any off flavors. The second reason is to allow the brew to clear and age a bit.
What comes into play after you rack to the carboy is beer clarity and gravity. I always sample my gravity when I transfer. Whe your beer is within the final gravity of the style of brew then it is time to bottle. If it is still too high then you should wait another couple of days/week (whatever is recommended for the brew).