3 of 4 added 35 characters in body

I figured it out. From the documentation: (0W8.

I found the following somewhat related answer on another SE site, to explain why I use a double-indent (i.e. (0W8 instead of (0W4) for continuation lines (i.e. Google and Oracle style guides encourage it).

WN    When in unclosed parentheses and N is non-zero and either
      using "(0" or "u0", respectively and the unclosed parentheses is
      the last non-white character in its line and it is not the
      closing parentheses, indent the following line N characters
      relative to the outer context (i.e. start of the line or the
      next unclosed parentheses).  (default: 0).

        cino=(0                    cino=(0,W4
          a_long_line(              a_long_line(
                      argument,         argument,
                      argument);        argument);
          a_short_line(argument,    a_short_line(argument,
                       argument);                argument);