Managed descent works fine, if you set up the right constraints. However, the fact is that you are rarely left to fly speeds and tracks that you programmed in the FMGC, so you need a feel for the alternatives.
The most useful metric I have in my arsenal is that a three degree glideslope uses 320 feet per mile, or near enough three miles per thousand feet. So if I'm told I have 25 miles to run, a three degree slope to the threshold should have me at around 8000' AGL. At 250 kts/8000 ft, I'm a little hot at that stage. At 6000/250 I'm looking good. At 250 kts and 10000', I'm definitely high and need to do something now to sort it out. And so on.
Oh yes, and don't think about the threshold - that way lies unstable approaches. Instead, think about 1500/160. Or 2000/180. Give yourself an earlier gate that you know if you make, then the last part of the approach will be okay.