Boiler safety valve is set to lift at any pressure, but not more than 3% above working pressure. Also the safety  Valve re-sits at 5% below working pressure.

 There are 2 safety valves. Both the safety valves are to be set at a pressure not exceeding 3% of the normal working pressure (stamped on name plate).

Let us say the working pressure of the boiler is 7 bar.

3% of 7 bar is (7+0.21) = 7.21 bar.

 which means, both the safety valves have to be set at a pressure not exceeding 7.21 bar.

 Apart from that, let us say one of the valves have been set at a pressure say 7.15 bar (assuming two digit decimal even though the practice is not referring to 2 digits), then the other valve must be set at a pressure range of 10% within this.

i.e the other valve setting must be 7.16 bar.

 I used two digit decimal only for understanding the precision of calculation.

The second safety Valve must be set within 10 % of the setting of the first safety valve.