Thanks Capn Bloggs
That was my mistake. I meant NM, not kts
Just want to have my own rule of thumb instead of 100% relying on the computer-generated DES path or "VNAV" in Boeing world.
Your understanding about green dot is right, but according my figures I didn't mean to use the green dot speed from TOD all the way down to landing. Normally speed is 300 kts, subject to CI and ATC requirement, until approaching 10,000' and then reduces to 250 kts. 300 kts needs longer distance than lower speed to reduce to green dot or S or F speed. That's why!