Given that the line is generated based on a destination and route, and clearly knows the vertical components of the drive, I've wondered whether the line is generated based on speed limits known for the route and the adjustment speed I've set in the settings. I find that if I set the cruise to follow the posted speed plus my additive factor (whatever it is) I arrive within a percentage point. If I pick up the pace, the line drops down a few points... if I'm stuck in slow traffic, the line moves up and I gain range.
To me, it seems likely that the speed adjustment back in the settings screen is factored into the calculation (which would make sense). I haven't dug deeper to see if I'm right...