When to flush?

[Missy]

Hi, me again, o.k so am at 35 days in flower and my purple bud fem seeds take 50-65 days until harvest. We're 35 days in now so the COULD be ready as early as Jan 2nd (I wouldn't be surprised as they produced pistils just 3 days after flipping). If that was the case I'd need to start flushing on the next watering right, but how do I know they're not gonna take upto 60-65 days? I obviously don't want to flush prematurely and deny 2 weeks of nutes and harm my yeild. So are their any signs to look out for that they're entering their last 14 days?
Thanks in advance