Reduce timing to reduce boost on rotrex

Was cleaning my engine bay today and looked like my pulley was a little worn..Thought that's strange as havnt covered many miles since getting the kit fitted a few years back which I supplied a brand new 110mm TTS pulley for as the kit had the 100mm one already..... Measured it and looks like it's 100mm. Worried about how much retard has been needed to stop all that boost.

So would retarding the ignition keep boost at a safe level as I know the 100mm pulley usually would make around 19psi on this kit

You won't reduce boost with timing, however, retarded ignition will keep you from knocking... if it was tuned properly.

As Dimitriy has said, ignition retard won't reduce boost pressure. On a turbocharger excessive retard can actually generate more boost as a result of increased exhaust gas temp. That obviously isn't a consideration for a centrifugal SC.