c-backslash-column

c-backslash-column is a variable defined in `cc-vars.el'.
Its value is

set-from-style

• This variable is safe as a file local variable if its value
  satisfies the predicate `integerp'.

Documentation:
*Minimum alignment column for line continuation backslashes.
This is used by the functions that automatically insert or align the
line continuation backslashes in multiline macros. If any line in the
macro exceeds this column then the next tab stop from that line is
used as alignment column instead. See also `c-backslash-max-column'.

This is a style variable. Apart from the valid values described
above, it can be set to the symbol `set-from-style'. In that case,
it takes its value from the style system (see `c-default-style' and
`c-style-alist') when a CC Mode buffer is initialized. Otherwise,
the value set here overrides the style system (there is a variable
`c-old-style-variable-behavior' that changes this, though).

You can customize this variable.