Ex Douglas Driver,
Now that I've had a look at your edits, the first segment distance consideration depends on the aircraft Type (gear retraction times) and the day ... better performance, more D1, lower performance, less D1.
For those who find this a bit strange .. first segment is that bit of the takeoff from 35ft to the completion of gear retraction .. if today it goes like a sports car, then you will be through 35ft fairly quickly, leaving a greater time for the completion of the (nominally constant) gear retraction cycle time and a corresponding longer D1 .. if, on the other hand, it's a dog .. then most, or all, of the retraction will be completed by 35 ft and there will be little or no remaining retraction cycle part to worry about ..
It follows that some aircraft don't have much of a first segment. The 60m/20m in your post could well be fine .. but some aircraft have 2000 ft plus D1 to contend with.
As this segment is very much a lower gradient than the second segment, runway slopes and obstacles in the immediate takeoff region become quite focussing for the ops engineering folk and, one hopes, the guys and gals in the sharp end.