George the expansion joints that I have been using have instructions on each joint.
They do not say anything about how to set them dependent on temp, but I have wondered the same thing as you.
What they have is a scribe mark that shows how far to insert the PVC, that mark seems to be in the mid point.
I imagine if they are all set in the mid point and installed every 30' as instructed they will be enough overlap to deal with higher or lower temps than the day of installation.
These new joints are only about 6" long overall in 1.25" raceway and only cost a little more than a coupling.
