Library /sys$common/syshlp/lse$clihelp.hlb SET, SCROLL_MARGINS, Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 top-line-count
    Specifies the number of lines down from the top of a window at
    which you want downward scrolling to begin.

 bottom-line-count
    Specifies the number of lines up from the bottom of a window at
    which you want upward scrolling to begin.

 %
    Optionally specifies scroll margins as percentages of the window
    height, rounded to the nearest whole line count. This is useful
    when you have a workstation with screens of varying sizes.