Our experience with several of each Hydroforce version and Sal's Spraymaster version is that the adjustment can never be trusted.
It got old, we went back to fixed dilution, other than occasionally changing out the yellow orfice restrictor in the chemical pickup tube.
In high production commercial situations we sometimes remove the check ball and spring, but we have shutoff valves on the end of all solution lines and you MUST then shut when finished or the tank will backfill with water. (useful when recharging it).
Doing that allows you to rock and roll thru big jobs without the heat slowing the induction of chemical into the line....or stopping it.