This is what Carlon says about it. I think this seems pretty much how most of us would install them anyway.
Setting Expansion Fittings Setting Position of PistonIf an expansion fitting was installed completely closed with
the piston bottomed on a cool day, there would be no travel
available when the conduit warmed up and expanded. If it
was installed in a maximum open position on a hot day, the
expansion fitting would pull apart when cooled. Thus, “setting”
the expansion fitting is very important.
In addition to the proper number and sizes of expansion fittings
chosen, they must also be installed so they can accommodate
the expected length change. This requires proper “setting” of
the expansion fitting. Most will be installed at a temperature
halfway between maximum and minimum; for example,
installation at 65°F with an anticipated maximum temperature
of 125°F and a minimum of 5°F. In this case, the expansion
fitting would be installed or set with the inner piston halfway
out. Expansion fittings are marked with a mid-point. If it was
a colder day, for example 35°F with the same expected
temperature change range, the inner piston would be set with
1/4 of its maximum travel out. If the temperature was hot
(95°F), it would be set with the inner piston only 3/4 way out.