One way is to use the FAP for all the calcs at all times. Just as an overrun does not start by the edge of the tarmac, an unstable approach begins much sooner than the threshold.
I find it helpful to check my gates earlier, and easily discover what goes and what goes not while you still have time to salvage the approach or abandon without the tunnel vision.
PROG page REF FIX to the G/S intercept point (worth checking WELL which FMS wait it actually is) and then
for instance: distance to FAP = 20 NM —>> you can kill 6000 ft.
FAP at 4000 feet <<==>> 10000 would be good. Compare to present ALT and see where you are.
That removes all ELEV guesswork, and works irrespective of the direction of your arrival relative to runway landing course. I.e for straight in you will need 3x to thr, if the opposite runway is in use then 5000 ft + 3x to thr is required mostly: all this becomes unnecessary with the g/s intercept point method. Sorcerer level 2: estimate the track miles to FAP, not just the direct line from REF FIX page.
As well, it gives a good understanding of what is the worst case short vector scenario. And if you are ok, then ask for it
Mentioning the energy difference between 320, 280, 250 and 210 kt is unnecessary here, same for the wind effect.
On 737 you could just use the VNAV (-300 ft), but on the Airbus - I find it not fit for the purpose.
2c.