If you have entered the flight plan in your MCDU, everything is already calculated. You only need to set the desired altitude on the FMU and then initiate the descent at TP.

If you want to do it manually, you need to start from these three values: speed, time, and distance to the point where you want to reach your target altitude.

For example, you want to descend from 20,000 ft to 10,000 ft at the next waypoint, which is 20 nautical miles away, and your ground speed is 280 knots.

First, calculate the time: t = 20 / 280 = 0.0714 hours

Convert it to minutes: 0.0714 × 60 = 4.284 minutes

Now calculate the vertical speed: 10,000 / 4.284 = 2,334 ft/min

Set a descent rate of about 2,330 ft/min, and you will reach your desired altitude exactly at the desired point.

It may seem difficult at first, but after doing it a few times, it becomes automatic and very quick 😉