For VS, just multiply groundspeed*5, be sure to round up or down.
A formula that always works for me is current altitude-target altitude/100 then divide by 3. Just gotta change the descent rate for every 20kt loss or so, always gets me there accurately.
Here’s an example if you’re still confused. You’re at FL410 and want to get down to 3000 feet. 41000-3000=38000. 38000/100=380. 380/3=126.7, so you start your descent around 126.7nm from your target waypoint/fix. Lets say you’re at 551kts and nearing TOD. Well, 551*5=2755, and that’s closer to 2800 than 2700, so you round up and go for a descent rate of 2800fpm. Obviously you’re gonna start slowing down, so once you dip below 551kts you’re gonna round down to 2700fpm, because below 550 and (eventually) 540kts, whatever value you multiply 5 by, will be closer to 2700 than 2800. Basically you keep decreasing descent rate every every 20 knots. So your descent rate would be decrease to 2700fpm below 550kts, 2600fpm below 530kts, 2500fpm below 510kts, so on and so forth. Hope that helped.