SE, there is a way to find out which one to use, but it depends on how slippery your case lube is and how much gets put on a batch of cases.
Use a standard shell holder (.125" height) then full length size cases and adjust the die height in the press until the fired case shoulder gets set back .002".
Then put another lubed fired case in the shell holder and run the ram all the way to the top and leave it there; do not lower the ram at all.
Using feeler gauges, measure the space between the shell holder and bottom of the sizing die. If a .004" feeler gauge just goes in, that means there's .004" between the shell holder and die.
The shell holder to use is one that's .125" plus .004"; .129" high. It is marked "+.004"
Check this page on Redfield's web site
Note the part that reads: "is clearly marked to indicate the amount it will decrease case-to-chamber headspace."
I think that should say the amount it will increase case head to case shoulder headspace. It makes the bottom of the shell holder further away from the die's shoulder. You really are not changing anything in the barrel chamber's dimensions. I'm gonna send this info to Redding and hopefully they'll correct the info on this page.
Full length size 5 fired cases and measure each one's case headspace with your Mic. If the average is .002" less than the average of what the fired cases have, then you're set to go.