When to flare?
Fortunately, this question has been answered for you by the people that wrote the book on your helicopter. Each helicopter has it's own uniqueness that means the when to flare and the how to flare are determined by flight test. Learn to fly your machine that way and only that way.
Go with an instructor and practice the auto with the numbers, until you develop your own visual cues. Then you can fly the auto with your head more outside the cockpit and the instructor confirming that your cues are accurate.
Once in the field, things may change. Do what training you can in the environment you work and discuss with the senior pilots how things change for them.
Now that you're in the field and not in a training environment you may find that the rule above, "[fly] that way and only that way", doesn't apply completely. In any case trust the experience of those who wrote the book, those who instruct you, and those who have flown in your environment and machine.