You should know the pumps max pressure in order to set the relief pressure. About 50 to 100 psi below the pumps max pressure.
Before the loader was installed, the hyd block relief pressure was set for the 3pt, and you also have the 3pt relief valve pressure.
Tractor hyd relief pressure may be higher then implement relief pressure.
The lowest relief pressure in the first valve will set the pressure for the rest of the system.
For instance the tractor relief could have been set to 2500 psi, but now you have installed a loader with a relief valve setting of 2000 psi.
That means that the new tractor relief is 2000 psi.