On Fri, 15 Sep 2017 16:48:59 +0700, John B wrote:

I've been thinking about chain wear, sometimes called chain stretch,
and have done a bit of research on the subject.

....snip...

So the question is what is the best system to use to avoid unnecessary
sprocket wear, assuming that sprockets cost more and are more trouble
to change than chains.

There seems to be three options. One, to use a ruler and measure from
pin to pin. Two to use a chain tool and measure from roller to roller.
Or three, to use some combination of the two.

Before I check the other answers: I use a chain measuring tool that goes
betweenthe rollers. This is on the assumption that what the cog teeth
see is the rollers, not the pins. I am assuming that the designers of
the tools (I have two, a Park and a Rollhof) took into account that the
tools measure two rollers at once, which may double the wear measurement
as the two rollers are being pushed in opposite directions. IIRC this
was Jobst's complaint about chain checkers.