Best point to harvest

It totally depends on your own preferance. I like to always look at the overall plant condition, if it's already fading hard and buds stopped swelling I might harvest at mostly milky, but if the buds clearly keep still swelling up I don't mind a little more amber if that means a higher yield. Generally 10-20% amber imo is the best. How fast it turns colours is quite dependant on strain, I've had some go amber a week after going milky, and some go milky halfway through flower but not develop any amber until a month later.