What should the input pressure be for a hydronic system?
I doubt that the 50 PSI pressure reducing valve is for boiler service. It is most likely to be used as a domestic water reducing valve. Boiler PRV's are all pre set to 12PSIG. That said, the boiler pressure should have a cold fill pressure setting is high enough to hold a column of water up to the top of the highest radiator. You can calculate that pressure by multiplying the height in feet from from the boiler pressure gauge to the highest radiator and multiplying the dimension in feet by .433. So for example, if the height of the highest rad is 25 feet then (25 X .433 = 10.825) the minimum boiler pressure setting. Then you add a lb or two for the minimum setting. Round up the 10.825 to 11 and add 2 psi for a cold setting of 13PSI.
Most residential boilers are rated at a maximum working pressure of 30 PSIG. Then the relief valve, sometimes called a safety valve, is set for the 30 PSI maximum .  Increasing the pressure above the cold fill setting does not affect the boilers efficiency. The pressure is just to make sure that the water level is held up to the highest radiator that is all it does.
Now, I am not trying to insult you but your question/explanation makes almost no sense in regard to boiler operation with over pressure, venting, cooling down, etc. Before you go any further, look up on U tube or other forum for how a hot water system operates.