Based on your response I am hearing you have an external waste gate. The opening pressure of the gate is set by the spring inside of it. So if its a 15psi spring (1bar aprox) then when the pressure on the signal line (vac hose from MBC) reaches 15psi it will open the gate.
If you would like to find the min pressure you can run simply run a vac line from the manifold or turbo to the side port of the EWG, this eliminates the boost control completely and runs "spring pressure". This will indicate the lowest boost you can run without changing the spring in the EWG.