You may be referring to the ejector(plunger) rather than the extractor(hook), because the spring tension on the plunger can theoretically push the case further into the chamber, giving a false reading on the headspace dimension from breech face to the datum line on the shoiulder of the chamber. With the ejector plunger and spring removed you can simply chamber a dummy/empty case and see how easy it is to lock up the bolt lugs, when it gets tight you have reached the maximum headspace dimension for your cartridge case that will still fit in your rifles chamber. Erik Cortina has demonstrated this on his custom F class rig, where the bolt handle either closes fully just on gravity or it hangs up and needs force to get it closed.[If I'm not mistaken his rifles do not have any ejector plungers in the bolts, you can easily notice him not only hand loading but hand ejecting in his videos)
The plunger will also affect the base to ogive/lands measurement and comes into play when you insert a modified case with the Hornady gauge. That is a different gauge than posters have shown here, rather"Comparator" in this context of sizing brass, generally refers to the gadget that you put on one of the forks of dial or digital calipers, they do not give an abs absolute measurement and are useless for comparing to SAAMI spec diagrams, as has already been pointed out here, those measuring tools only allow you to compare two cases. You do not zero it against the fork, but rather against another case. For instance you put in a fire formed case, then zero on that baseline and it will tell you how many thousandths you are bumping back the shoulder on a resized case.
Used properly those comparators will be yielding numbers like"-0.002" never "1.638"
Whidden dies, incidentally, come with a comparator that fits the caliper fork without the red adaptor. I am also a fan of the RCBS precision case micrometer.
And if nobody's said it yet if you plan on hunting with that rifle in cold weather it's always a good idea to completely disassemble and the grease the bolt assembly and use dry lube when reassembling. You don't want grease to harden from the combination of temperature changes and condensation.