Now that is impressive! -15c. If your at 50-days your estimate may be more in line. Go by what the plant tells you. I did 5-autos and had some ready for harvest at 100-days and some ready at 130-days. The trichomes will tell the story. As I kept my eye on them it became very apparent when they were ready to harvest.
The best moment for harvest is when your pistills are in 70-90% orange/red. But the best way to check the THC content is by magnifying glass and analising trichomes. Glassy trichomes are low in THC and CBD, cloudy/whity trichomes means that you will get more like sativa high and more amber trichomes means you will get more indica high. The best place to harvest is when 50% of your trichomes are amber and the rest are cloudy white, it’s where your flowers has the most THC content and the highest yield. When you miss this spot your ladies start to overmature and THC is transformed into CBD. My preference is to harvest just slightly before 50% of trich change to amber because I want to get the biggest yield and I like more sativa like high.