There is not enough root development for the prolif to be able to support itself, so a portion of the scape is left to give the prolif a good stable base to hold it up and keep it from moving around (wind, pets, you etc) and preventing the small roots from losing contact with the soil. So whatever amount of scape is necessary to provide that steady anchor should be the amount of scape left when planted. I don't think the prolif at this point would be getting any nourishment from the scape after it is detached. So the height and amount of leaves on the prolif can be a determining factor as well as the type of soil it will be anchored in . Heavy clay soil would not need as big of a piece of scape for an anchor but very light sandy soil would need more.
