think thats the "dodgy" way of doing it as the AMAL valve or ECU controls it all doesnt it? On my r5 turbo, i just did that. But if your mate has turned the boost down that way, all you have to do is shorten the rod and keep trying till it holds the boost it was at before. Think someone told me 1 full turn is about 1psi. (that was on the Renaults though)

HTH