I was one of the earliest users of the 123 ignition in a Z car and so have quite a bit of experience with it over the years that I have used it.
I can also tell you that I had a similar advance problem at idle with my initial install. If I remember correctly, and according to the install instructions, everything needs to be at TDC on the engine. You install the 123 distributor, apply power, and turn it until an LED turns on and that's where you are supposed to lock the distributor in place. However, after some trial and error, I figured out that at idle I was at about 30 degrees advanced, and this was with a flat 0 degree advance programmed into the 123 distributor. I ultimately concluded that the distributor was not rotated to the proper point for a 0 degree advance, despite the fact that it had been installed properly (with the LED) as per the instructions. So what I ended up doing was unlocking the distributor, ensuring it was programmed for a 0 degree advance (up to about 1500 RPM's), and rotated the distributor until I was at 0 degrees as measured with a timing light. I then programmed in some additional advance at idle (I believe about 12 degrees) and double checked it with a timing light to ensure it agreed with the programmed values.
So for some reason the LED install instructions did not work in my application, but by setting the initial timing of the 123 like you would a normal distributor, I was able to get it to work as expected. And the ease of changing timing curves is awesome.
In any case, hope that helps. If you have any other questions on this just let me know.