no equation bro, all you can do is stare at all your budsites on a daily basis n speculate on how fat theyll get. It all depends on node spacing/how big the calyxes will get, how many budsites overall your plant has, n all that good stuff. if people could predict what their final yields will be without growing the exact same grow ten times over, things would be changed a little bit differently with how growing cannabis is done today.
But yeah, depending on the strain, your plant will get at least 2x bigger, but in optimal conditions its plauseable to get triple the size by the time the stretch is done. Like was said above, lighting plays a big role, and so does temps, but as long as your not on the extreme end one way or the other, your plant is gonna turn out badass. I mean, it already looks fuckin awesome dude. you just started 12/12 tonight?? n its already started flowering, so i mean, you could in all actuallity be well into "the stretch" right now as we speak. In the last week has your plant just started to turn into a monster, everyday just gettin bigger n bigger? if so, then your into the stretch already, and youve probably got another good week of the same type of growth before she stops and starts to focus solely on bud production, and crystal or resin production. Bottom line is man, your shit is already pretty as hell, and your gonna end up with one beast of a plant dude.