Do what works best for you, the usual advice is to look far down the runway from the start of the flare.
Please also note the following : the FCTM height is 30ft, then they go on to listing all sorts of conditions when the flare height should differ.
First of all, obviously if your mass is lower, all little buses having the same wing, it will make for a slower speed hence lower descent rate. Then, the wing being the same, for the same amount of pitch up you will get the same amount of lift. So you will get a stronger flare with a lighter aircraft.
This explains why the height may vary as much as from 20ft for a light 319 to 40ft for a heavy 321 (maybe coupled with the differences in flare speed as well)
(Airbus try to make you believe that all planes from the family handle the same, from 318 to 321, and land the same with the same technique, however that is just plain wrong.)
Also, this flare height is a bit early (except 321) also to help TRI's monitor the situation. Since they don't have a moving control column to effectively monitor the FO's actions, at least they can have a split second to react if they don't see anything at 30ft.
At the end of the day, a slow flare from higher can very well make the same result as a fast flare from lower (that is, touchdown distance and vertical speed at impact). It's up to you to get a technique that works all the time and to know how to adapt it to specific conditions.
Same goes for thrust. Even though airbus recommends using auto thrust, you can very well use thrust to require less flare and conversely.
Please also note that the flare height, for the same flare "force" (g-load actually) changes with the square of the vertical speed.
So vertical speed is really your main parameter.
Note that if you have an unstable final descent, you may touchdown very hard if you flare the same as usual with just 200 or 300 more fpm just prior to flare. Just to have an idea, if the standard technique is 20ft height (no margin) for 700fpm, then having 1000fpm at 50ft will require to pull at 40ft. Here you can see the true relevance of the "sink rate" callout.
If you're on a 4° slope instead of 3°, with the correct vertical speed, you should pull at 35ft instead of 20.
Conversely if you pull slowly from 40ft, you reduce your vertical speed. If you reduce it to, say, 400fpm, you now only need 7ft to stop your descent. So that's an excellent way of floating above the runway at 10ft and then having to descend again.