Nigel,
By disconnecting the SCV you are asking for full pressure. Readings I have from a Nissan Pathfinder on a similar system are 1.5 volts shows 300 bar on live data, and 2.0 volts shows 600 bar, and appears to be the maximum the system defaults to when it goes into limp mode. Measuring duty cycle at the SCV, having 'scoped a good signal, duty cycle decreases to increase pressure,
hope this is of some help,
Peter