Have a Chat, presumably you are into wind, wings level and at the recommended auto speed (probably 50-70 kts depending on type), hopefully you have been in that configuration since at least 300 'agl to allow the RoD to stabilise at its minimum.
Your flare height will depend on aircraft type (R22 about 40', Sea King/Lynx about 150', most other things like Gazelle/ 206 etc about 100') and is primarily a function of RoD - the faster you are going towards the ground the sooner you need to do something to stop you hitting it hard.
Learn to try and judge the flare height using external references(heights of trees, houses, sheep etc) rather than slavishly using the radar altimeter if you have one. The concept of waiting until the groundspeed seems excessive is an interesting one but how do you assess excessive? When you are hurtling towards the ground it all seems to get far too quick, far too quickly and you run the very real risk of leaving it too late.
The more EOLs you do, the better you will learn how to not only judge the required flare height but also how to vary the flare depending on wind strength and AUM. A heavy aircraft in light winds will need more flare started higher than a light aircraft in a strong wind.
There are many variables to consider when aiming to carry out a
textbook EOL but the technique detailed in the previous post should stand you in good stead. The British military teach 'Flare, Check, Level, Cushion' as the 4 main parts of the EOL with the Check element only being left out for low inertia rotor systems (R22 ) or big heavy aircraft where you need all the Nr to cushion the touchdown.
Good Luck