Hey.
Just like you, I am also a newbie and have found this technique EXTREMELY useful.
I put in the GS intercept point, or on NPA the FAF on the MCDU prog page to give me the distance.
I use the altitude to lose x 3 + 10
I add or subtract some more depending on headwind or tailwind.
I calculate from platform altitude since that's the point I have on the prog page.
This way you will either end up spot on, or slightly low, which as newbies is safer than ending up high.
I have found that with more flights, consistency, calculation gets better, I take into account traffic, etc.
Even then sometimes I am not certain and I will ask the captain what he thinks.