One formula states that the total area (in square inches) of the safety valves should be:-
Total heating surface of boiler (in square feet) times (K/p+15), where p = working pressure,
and K = 1.25 for coal fired or 1.5 for oil fired.
( This appears to work out best for full size boilers. )

One safety valve should be set at the designated pressure and the other, very slightly lower.
(About 2 to 5 psi, or a maximum of 10% of the working pressure.)
Some authorities recommend that the second safety valve should be larger than the first, and it may be
advantageous to make it a "pop" valve to drop the boiler pressure as speedily as possible.

You should check the requirements of authorities, insurers, clubs etc. in your area, as some require specific settings.

In any event, safety valves should be checked at the start of each days running.
Run the boiler up to full pressure and ensure that the valves open (and close again) and are not stuck in the closed position.

There is one overriding test of the correct capacity of safety valves, which is, that they MUST be big enough to relieve the pressure, no matter how hard the boiler is steaming.

For UK club certificates, such a test is mandatory.
In other cases, it is strongly advised as it is the only true measure of the safety valves.

The majority of recent designs have safety valves that are sufficient to meet this and any other reasonable test. Some of the older designs, especially those of  half-a-century or more ago, specify safety valves, and passages that are too small to meet current requirements. It is worth checking to ensure that the safety valve is big enough before making it, and in any event, the test above should be performed in any case.