When to start flushing?

Ok so I know u should flush for atleast a week. But how exactly do you know when to start the flushing?
I know you know when to harvest by the look of the trichomes, but how can u judge when to start flushing so that when ur flushing is done, the plant will be ready to harvest?

watch your trichomes.
I wouldn't worry to much about hitting it right in the head
when they look like there about 7 to 10 days away from harvest
start your flush. if it 5 days oh-well if it's over 10 no biggie.

Reference Link:http://growingmarijuana.org/general/tip-2/