As soon as 100% of the trichomes are milky and some begin to become amber, you can start the Flush. I only grow in potting soil and 15 days is the minimum required if you want a good taste.
Even a little more, like 20 days, is very good with potting soil. Everything depends on trichomes at this time.
20% / 25% amber is a very good ratio for most hybrids
Good luck for the end my friend