Scrolling units are typically set while processing the WM_CREATE message. It is convenient to base the scrolling units on the dimensions of the font associated with the window's display context (DC). To retrieve the font dimensions for a specific DC, use the GetTextMetrics function.
In the example in Example of Scrolling Text, one vertical scrolling unit is equivalent to the height of a character cell, plus external leading. One horizontal scrolling unit is equivalent to the average width of a character cell. The horizontal scrolling positions, therefore, do not correspond to actual characters, unless the screen font is fixed-width.