Standard Airbus technique if HIGH on the profile as indicated by your magenta deviation 'donut' (yes i know its not a donut anymore on the latest revision of the fmgc, before anyone points it out!) is OPEN DESCENT and increase SELECTED SPEED, if you remain fully MANAGED, deploy 1/2 speedbreak in order to meet the 'rejoined profile' down arrow, either works fine, but the OPEN DES method is more decisive. If low then VS and SELECTED SPEED, (managed speed can be used, but some trainers like to see that you are accepting control over the FMGC by Selecting your speed as well as your RoD.) When back on the profile, go back to fully managed. The only time you really need to calculate the Distance vs Altitude is when on vectors and you have cleared the FMGC to only show the Approach from the Final (or Centr) fix to the GA point. The donut is still there to help you, but backing it up with some mental arithmetic is good.
The Airbus concept is smart aircraft for dumb pilots and largely you can use this philosophy safely to make your life easy, which suits me fine. BUT, what Airbus don't say on the tin is that buried away inside your clever aircraft are lots of traps for the unwary in the form of computer glitches and oddities of logic. Your mission is to be ahead of the aircraft and have a really good understanding to the FMGC and the automatics, cause if you don't some dark and stormy night your friendly Airbus will bite you on the backside.