You have a good handle on it. They double in size from one phase to the next generally. Sativas can even triple so its important to know what your strain will do. A website seedbank.eu (I believe, maybe seedbanks.eu) has good info and will usually tell you what to expect for a stretch. Most growers flip at around 4 weeks. This is a completely reasonable time frame. Training will draw the veg time out and if you have lots of space and strong lighting, you might wait even longer. Good luck!