2 weeks is a fair average too, so yes, go from the date ~2weeks AFTER you flipped to 12/12. That should give you a better idea, and usually if a strain description says 7-9 weeks, expect 9 to be closer to what you expect.
What Gov said about them turning quickly is correct. I looked at my 2 plants that had only 2-4 on a sample clipping 4 days ago to what I think is about ready to harvest today. My Sativa strain Vortex did not gain that many but has had amber trichs for about 10 days. the hybrid Angry Ape (AK47XGrape Ape) went from only 1-2 to about 40-50% in the 4 days.
My 2 plants. the bathtub tiles are 4" for a reference. Angry Ape on the left. Vortex on the right.