This is my technique for the Descent in the Airbus.
First off I take the platform ALT (lets say 3000ft) and insert the FAF in the PROG page. So I know at this point I need to be at 3000ft With flap 1 or 2.
Now I take my current alt, Let's say i'm at FL360. then I subtract 3000ft (platform alt) which leaves me with 33000ft 33*3=99+5(for slow down)=104NM. Now you have to take account for Wind, I use a rough estimate of 30kts of tail will be around 10nm so 33*3+5+10=114nm, So around 114nm from the distance which is on the PROG page I will begin my Descent.
Me personally will do this Cal every 3000ft just so I have some situational awareness. Be advised that sometimes you have to think outside the box.
Example was yesterday I was flying to Riga, The star there is pretty messed up and you hardly ever fly it ( short cuts or radar vectors)
On the ND my arrow was estimating when I should begin my Descent, But something didn't add up. It took into account the whole STAR and was calculating the whole distance, I just looked at the ND saw the Airport was 120nm away, times by 3 should be around fl360 and calculated from there.
I am also quite new on the Airbus have 1000hrs on type now, If I can help you anymore please feel free to drop me a PM.