The "Idle-up" diaphragm's job is to increase idle speed to compensate for loads that would cause the engine to stall or stumble. A properly adjusted one will simply aid the engine in maintaining the correct idle speed when these loads are present.
With the AC off, set your idle RPM using the idle screw adjustment on the throttle body. Then turn on your AC, wait for RPM's to stabilize, then set the idle speed again (to the same spot as before) using the adjustment screw on the "Idle-up" diaphragm. Tim