I have a boiler that's operating range is 12 to 30psi.The documentation says that it loses "efficiency" at the low end of of the operational pressure range. When looking at pressure reducing valves typical preset valves are 50 or 15 psi. My thoughts are if you reached an over pressure situation and the system vented back down to 30, it would have to cool off and drop back down to the inlet pressure to start to refill the system. In that case 15 seems like it could take a while for the system to adjust for the venting. On the other hand a 50psi valve can be adjusted down to 25, but operating 5 psi below max by default seems like a mistake. What should be considered when selecting the correct valve?
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.