class RuboCop::Cop::Layout::FirstArgumentIndentation

def column_of(range)

is simple. For ranges with line breaks, we look a the last code line.
Returns the column of the given range. For single line ranges, this 
def column_of(range)
  source = range.source.strip
  if source.include?("\n")
    previous_code_line(range.line + source.count("\n") + 1) =~ /\S/
  else
    display_column(range)
  end
end