can I use the controller to bring the 21 volts from the brick down to 18-19 before going into the TIU? Also what channel would be used in the TIU, variable or fixed ?
As I said above:
Under DCS, use an MTH #50-1017 TIU/Barrel Jack Connector to connect to a TIU variable channel, and use the variable channel to adjust the voltage.
In this case, you would not use the Z-Controller.