Brian
You are correct that the bleed is an either open/closed system. It operates as a function of engine speed (rotor RPM) and CIT (Compressor Inlet Temperature), which has a maximum allowable limit of 427°C. As to a fixed percentage, I’m not so sure.
I don't think it can be a fixed percentage. Bleed open/closed is the equivalent of Concorde's secondary air doors (between intake and engine bay) being either open or shut. Once open the bleed flow would be controlled by the flows in the nozzle as explained in my other posts. It would vary with primary jet nozzle area for example, with bigger bleed flows possible at reduced Aj