Unlike many manufacturers, we have our own private production facility and have complete control of our build process – meaning we make bomb-proof boards. We do not have an upper pressure-limit on the boards, but beyond 25psi, you are only increasing the pressure without increasing the stiffness.
The more pressure you have, the better the boards performance will be, which is more important in heavier conditions or with heavier lifesavers/casualties.
As a rule of thumb, the absolute minimum we would recommend is 15psi, but for operational purposes, we would  advise 18-22psi.