The timing of the flare is a judgement thing that can only come with practise.
The auto descent speed is important to provide enough rotor thrust at the bottom, both during and after the flare, to stop your ROD for the touchdown.
However, the PRIMARY reason for the flare is to reduce your groundspeed for the touchdown. If you are coming down vertically in auto at zero speed, or doing 50Kt with a 50Kt headwind, you don't need to flare for the touchdown do you?
My advice is to commence the flare when the groundspeed appears excessive. As you get nearer the ground, for a constant groundspeed, the PERCEPTION of your groundspeed will increase. (Doing 450KT in a jet at 40,000' the groundspeed seems slow, but do that speed at 50' and it feels fast indeed).
It is like saying to yourself "I am going to hold my speed until the last possible moment before I MUST flare to lose the groundspeed". If I don't start putting the brakes on now, I won't be able to stop in time. Like coming up to the stop sign in your car and saying "not yet, not yet, not yet, NOW I have to start braking or I will not stop in time".
Flare too soon and you will have too much height at the end and probably hit hard. Flare too late and you must accept an excessive touchdown speed or hitting the tail.
Don't fixate on the ground, roll up your sleeves and work fast and scan. Try jugding when to stop for the stop sign by looking at the first bit of road you can see over the bonnet, rather than down the road a bit. Which do you think is easier, and gives you better judgement?