Great question — answering as the UphillTech team:
You’re essentially right. Crankbrothers has a good diagram of measurements of Highline 3 droppers here . From there you can see that the minimum extended length of the 80 mm dropper is 125 mm. This is the length from the bottom of the collar to the saddle rails, also known as stack height (e.g. by ParkTool ). What Crankbrothers calls minimum stack height is the distance from the bottom of the collar to the saddle rails when the dropper is at its lowest position. As you’ll notice 47 + 80 mm is 127, which is 2 mm taller than the minimum extended length. Perhaps the raise is only 78 mm or perhaps these numbers are measured from slightly different spots. If you want to figure out how deep in...
Full answer by EyeBrown: https://bicycles.stackexchange.com/a/100646 (CC BY-SA 4.0).
Need it done for your business? UphillTech.