The Hornady Headspace Comparator kit is the right tool to use. The correct expression is bumping the shoulder back, not bumping the neck.
The goal is the bump the shoulder back .001-.002.
Use the correct gauge for your cartridge (I believe the one labeled 400 is the one you want for 6.5 creed).
Take a measurement on a fired case.
Then turn your FL sizing die a tiny bit. Run the case through and take a new measurement. When the measurement is .001-.002 shorter, then your die is in the correct position. Locked the dies retaining nut to that position.
For semi-autos you want to bump it back a little more, .003-.004.
It might take you a few cases to get it just right.
Hope that helps!
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