Chain gauges that measure between rollers measure mainly roller wear which is not related to chain elongation and much less important. These gauges will have you replacing chains much more frequently than needed. As already suggested by others, use a good ruler (a steel machinist's rule is good) to measure 12" (center to center over 25 pins) to determine chain elongation. The rule is to replace the chain at 12 and 1/16". I choose to replace my chains at 1/32" of elongation to reduce wear on cassettes and rings. This usually occurs between 2500 and 5000 miles on the chain.

FWIW, I have discussed this issue with one manufacture of chain gauges and they agreed.