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Try 3 X the altitude to lose...then add 10 miles...

For this example, 3 X 33 = 99 plus 10 would be 110 miles. Your airplane may vary in it's profile, but it's a good rule of thumb to start with.

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I agree with MDpilot,

3 times the altitude loss works great. Divide the altitude you need to lose my 1000. (10,000' = 10) Then multiply by 3 (10 x 3 = 30 miles). This means you start down 30 miles from your waypoint.

This is only part of the equation though. You just figured out how far to start down, now you need to know how fast (VS) to come down. In something like a King Air, 1500 FPM works well as a ballpark number. When you get into jet speeds in the descent, that figure moves up to 2500-3500 FPM. Both of these examples give you approx a 3 degree descent angle.

At this point you start to consider the deck angle of the plane during the descent. If 3000 FPM is too steep (for pax comfort), then you add the extra 10 miles MDpilot was referring to (30nm + 10 = 40nm @ 1500-2000 FPM).

In a light twin/single doing 120 kts, to lose 3000':

(3000'/1000) x 3 = 9nm. You would need to descend approx 667 FPM to lose 3000' in 9 nm.

300'/nm descent = 3 degrees.