Altitude to lose / 3 = how many minutes it will take you at a 3000 fpm descent.
GS/60 = miles per minute.
Multiply the two numbers, and that's how far out you should start down for an idle descent.
It sounds like a pain in the butt, but after you get used to it, you won't need the FMS and it'll usually get you within 5 miles or less of the fix with time to level off. You have a little extra time because your TAS and therefore GS decreases as altitude does.
I know- I think too much, but it does work and you don't have to worry about wind.
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