If you have a bolt action and the cases have already been fired in that rifle you really don't need a neck sizer die.
Here is a method that will work.
Set the sizer die up for full length sizing.
Noting that the industry standard for dies is the 7/8" x 14 tpi, one full turn is 1/14" or 0.0714".
If for example you have a 30-06 with a nominal neck length of 0.383" and you only wanted to size about a third of it, say 0.130", you would then turn the die out 0.253" (or about 3 1/2 turns) to achieve this with your regular size die.
You will have no trouble seeing the die mark on the brass and in this senario it is highly unlikely that the body would be touched or the shouder set back.
Will sizing 1/3 of the neck be enough?
Likely!