The helpful thing my instructor told me to help me judge the height of the flare was to imagine sitting at the front of the top deck of a double-decker bus, and to flare at that height.
He also told me to switch my focus to the end of the runway, but the double-decker analogy really helped give me a concrete idea of how high to be.
The real problem I had with the flare was overdoing it and having to go-around. I still tend to be a bit twitchy on the flare, having to make very small adjustments following each pull back on the yoke.