When to initiate the descent?

Hello,

I'm playing MSFS 2020. FlyByWire A320, on the tablet you have the option to specify how high you're flying and the altitude of the airport under "Performance".

There a number is then spit out, such as "80 NM"

So I have to start the descent 80 NM from the runway.

My question:

The airway points can also be 120 NM long, the next one might be 25 NM long, the next one 10. How should I calculate that?

So that I really know and am sure that I am now exactly 80 NM from the airport, I have to descend now.

Since the airway points are still there and are sometimes long and sometimes short,

How can I be sure, for example, that I am now 80 NM away from the airport?

How do you get this in combination?

To initiate the descent appropriately.

Thanks in advance.