A line-length is an expression that specifies how long to draw a line segment. The value can be either absolute (ex: "1.2cm", ".5in", "0.5*circlerad", and so forth) or it can be a percentage value (ex: "85%").

If the percentage value is used, the basis is usually the value stored in the "linewid" variable. However, for a case of either:

…then the percentage refers to the current "lineht" value instead. The "linewid" value is always used for headings even if the heading is "0" or "180" or "north" or "south".

In most cases it does not matter whether "linewid" or "lineht" gets used for the percentage basis since both variables have the same initial default of 0.5in.