This is only my opinion, but:
Just estimate it like Hemp is saying, and use that as your guide. In a couple of weeks, start looking at your trichomes and switch to your clearing/flush regimen
once you are satisfied that the trich's are getting close to where you want them. I've landed on a 7-10 days as my
goal for flushing, but it's really tough to tell when they are going to ripen up very far in advance (for me anyway), so I have to "punt" frequently with this.
Wait 'til it's just about where you want it, trich-wise, and then start your flush regimen. In the end, even if you only get a few days of flushing, so be it. I absolutely guarantee you your buds will still be super yummy as long as they are dried and cured properly afterwards. How often do you get a guarantee like that? I've had really tasty bud that was hardly flushed at all, and there are people that think flushing is over-rated to begin with. Make your best guesses, and it WILL work out fine. Like I said - guaranteed!
Oh, and they will bulk up more in the next few weeks, but I'm a little uncertain if the LED will ultimately give you the bulk you want - you'll have to decide that in the end.
Good luck my friend!