RE: Best digital calipers to use when doing an extruder calibration for linear advance?
Since you asked for brand names, I don't think you will ever regret buying a Starrett or Mitutoyo (which Joan also mentioned) caliper. A good quality micrometer (as also previously mentioned) is a good addition to have as well. I render no opinion whatsoever as to whether you NEED one of these brands to do what you are wanting to do. Only that spending extra to get quality equipment is a decision I have rarely regretted.
RE: Best digital calipers to use when doing an extruder calibration for linear advance?
I have multiple cheap calipers and a Mitutoyo for the good measurements. Where I find the difference is in the alignment of the jaws for internal and external measurements between the cheap ones and the Mituoyo. 99.9% of my measurements are the same between the two calipers for external and close to the dial. On the other hand, for 3D printing, a 0.05mm error is as accurate as most lathe jobs I have ordered through work. It is less than 0.002 inches for those that don't do metric.