When do I start pre-harvest flush?

I tried to search for this, but didn't come up with anything.

I have 9 plants growing all from seed and all 51 days old from seed.

Started 12/12 light 2 weeks ago.

3 of my plants are Buddha Red Dwarf Autos. They started showing pistils 16 days ago.

I've read others harvest Buddha Red Dwarf Autos at 76 days old from seed.

If that is about correct, I would need to start flushing in about 10 days with just H2O to get a good 2 week flush.

Or do I need to see that some of the trichomes are turning cloudy and/or going dark?

Pics are of the crop and a closeup of one of the autos.

I just anticipate, like when I'm going to cut in 10 days to 2 weeks, I start on only distilled water, so you are a long ways from that point.

Ok, so since this is my first grow, I have no base point on when to "anticipate" when to cut. Any guidance on what to look for?

If it's the first grow then screw it, doesn't matter if you do it wrong. For this go, simply grow it out, you'll get an idea of how it grows and how it starts to change closer to harvest, then you have a reference for the second attempt on when you should start flushing. I'm coming on 2 years growing and i still do it wrong, i've had a girl flushing for a month odd as she just refuses to ripen up, live and learn

I flush for the final week of flower.
When's the final week?
Most cross or indica strains take 60 days (8.5 weeks) to flower. You can start flushing at day 53 and if some of your plants aren't quite finished yet, let them grow! Just keep flushing for another week or two.... they will still finish without the nutrients.