Another advice:
It's a feel thing and not a mental calculation.
I'll give an example, for Boeing.
At 20 nm to go, aim to be in e.g. 5000 feet AFE at 250 knots. With 3x height you now have 1000 feet leeway to slow down. Plus you most likely have an intercept turn as well. You can be higher if you have strong headwind or a sharp turn.
Break the RoD to start a deceleration. Now it's just a matter of adjusting RoD to facilitate the extension of the flaps. The experience comes into play in that, you decide when to extended the flaps (thus increasing drag), according to the progression.
Goal is of course to arrive at approach flap setting and according speed at the FAP, and at the correct height.
If it doesn't fit correctly, you can either arrive at a wrong height or wrong speed. Always arrive at the right height, and choose speed as your variable.
1)
Should you be slightly slow, add a little thrust, while maintaining and during the continuous descend, before reaching the FAP
2)
If you arrive at the FAP a little too fast, extend the gear a little earlier.
Easy peasy... Don't make a craftsmanship into rocket science.