It took me a while to get my rules of thumb in the types I have flown.
Then I use those and also I monitor my actual gradient, by looking at my rate and both GS and TAS. In this manner I see if things are going well, because sometimes your descent is just steeper than others, and who knows why… Atmosphere is complex, air goes up and down, you know.
In the 320, FLx3+6 was accurate below 10,000, but some models had a higher idle and in these FLx3+10 was more like it. A321 I can't remember, too long since I flew it. A330 is FLx4 plus plus, it is a glider!
But don't worry, just make your descent and if you get low, reduce V/S as required, even if it is 200 fpm. Then you can boast that you did a CDA because you didn't fly level
