This is a major reason I discourage people from counting flowering time from the 12/12 light switch. The plants don't instantly start to flower, they have to reach the point they are able to once their hormones reach the appropriate levels. Thus flowering starts once the plants start forming flowers, just like all the outdoor growers.
I would simply jot down both and keep this data in case one grows it again. Keeping track of the grow, noting all the change of the variables, it's something that can be recommended to novices, too, because then you have a steeper learning curve. But you're right, these guys that say look I'm in flowering week 2 but the plant just starts to stretch - then they cut in week 7, because that's what the ad said...
so BTT:
Seedling/Cuttling: 1-2 weeks
Veg: from the first true leaves on - seams reasonable
Flower1: 12/12 11/13 10/14: transitioning into generative growth begins at the hormone level only. The plant still grows like in veg. (1-2 weeks to my exp, but I use FR)
Flower2: Stretch - actually
increased vegetativ growth (allows for a very high nute concentration, or you shun a little to stiffle the vert grow). The new growth of the top stems comes out somewhat differently, hard to describe, like more finer leaves.... that's for me somewhat the first sign of different growth and thus, here flowering counting begins. The stretch may end differently on sativa plants, like being much longer...
Flower3: shoots start growing into intitial buds and develop sugar leaves
Flower4: buds start to grow long and develop into colas, this type of cola is more seen on sativa whereas in indicas the dont stretch that much
Flower5: bulking up, here they get dense and gain some weight
Flower6: ripening, not really anymore gains in weight but some allow the crop to age or get old to get to their trichome profile. Some may omit that completely when they only care for weight... some even cut even more earlier when Boss said so it would bring more bucks this way... XD
just my opinion here and I'm pretty sure there are many more ways to categorize the same thing.