One thing nobody has mentioned explicitly: IRL, you practically never calculate your descent all the way to the touchdown point. In an airliner, which is where descent calculations matter the most, you will practically always fly IFR, and use published procedures for the arrival. You plan your descent not to the touchdown point, but to the start of the procedure, which will typically specify an altitude to fly.
For example, look at this chart:
https://nav.vatsim-germany.org/files/ed ... H_EAST.pdf - it says to cross the ASPAT fix between FL240 and FL280, so that's what you aim for in your descent calculations. Subsequent fixes on the STAR specify further level restrictions, and you need to plan your descent between those to comply with them.
After the STAR, you either fly a transition, or a published approach - in both cases though, similar altitude specifications are provided. For the final approach, a more specific altitude profile will also be provided - the usual procedure is to maintain an intercept altitude (typically something between 2000-3000 ft AGL) until intercepting the glideslope.
As for the descent calculation: I like to use "operational descent" mode for this (it's called "FLCH", "Flight Level CHange" in the Boeings, "OP DESC" in the Airbus IIRC - but it's the same thing): this is essentially a mode where the autopilot maintains airspeed with pitch, and the throttle is set to idle (either manually, like in the CRJ700 which I fly most, or automatically by the autothrottle). This produces the maximum descent rate in clean configuration and at the selected airspeed. In the CRJ at least, if you reduce the selected airspeed while in op descent mode, it will fly level to bleed off speed before continuing the descent, which is exactly what you want, usually. I'm sure the Boeings and Airbuses can do something similar (though since they have full autothrottle, it's probably even easier). For the actual descent calculation, I use a little spreadsheet based on actual performance of the simulated aircraft. It's not perfect, but the basic idea is this: first, we estimate the expected average ground speed through our descent. At a constant airspeed of, say, 280 KIAS, it's going to be somewhere south of 400 kts at cruise altitude, let's call it 420, and maybe about 300 KIAS at FL100; so a crude estimate would be 360 kts. Then we'll take the descent rate we can achieve at 280 KIAS (off the top of my head, about 2000 fpm); divide the total descent (e.g. FL300 cruise - FL100 target = 20,000 ft) by that number (in this example, this gives us 10 minutes of descent). 360 kts is 360 nautical miles per hour, or 6 nmi per minute. So 10 minutes of descent at 6 miles per minute equals 60 nmi, and we'll initiate our descent 60 miles out from the target fix.
This is a fairly good conservative estimate that gets me down comfortably in due time, and doesn't require speedbrakes, which is nice, because speedbrakes basically cost more fuel (you could have initiated the descent earlier, which means you have been burning too much fuel by staying at cruise altitude too long). I basically reserve speedbrakes for when ATC assigns a descent that I can't make without, doesn't clear me for the descent in time, has me fly fast too long (the CRJ doesn't slow down easily), or when it turns out I misestimated my descent calculations.