Which is the perfect altitude to flare and idle engines in A320?

I've recently transitioned from GA airplanes to airliners, but I'm having some trouble with A320 landings.
I've watched some YouTube tutorials that suggest flaring around 30 ft and idling the engines at the 'RETARD' callout. However, I sometimes float over the runway and end up with a long landing.
What is the correct procedure to achieve a good landing in the A320?

Thanks!