The idle bypass only has a very minor effect on the fuel mix, and only on the idle mix. How are you determining the running rich? If it is indeed rich you will want to 1st verify the ECU temp sensor is tracking the temp curve, see the FSM for resistance measurements to temp reading. Use a thermometer in the neck of the rad to confirm your temps. Take the readings AT THE ECU 36 pin connector. 13 and chassis ground IIRC. resistance in the connectors both at the sensor and the bullet connectors on the sub harness must be clean and free of corrosion. A high resistance will result in a rich mix.
If that check you need to confirm fuel pressure, should be about 36-38 psi with the engine off pump on. With normal manifold vacuum (about 17-20 in hg) you should see around 29-31 psi (engine running idling after warmed up). The fuel pressure should track with vacuum, so as you open the throttle quickly the fuel pressure should momentary rise, the drop back when throttle is released. higher vacuum less pressure is the key.
Depending on the outcome of those there are more things to look at, but those will be the problem most of the time, assuming no one as monkeyed with the AFM spring settings.
As to the orig question about the bypass, I had some luck with setting it by using a vacuum gauge on the intake manifold, letting the engine warm up an settle down, making sure everything else was right (timing/fuel pressure/temp sensor) I found that I could tweek that adjustment to max out the vacuum (best eff of the engine at idle indicated by best vacuum I could pull) I then leaned it some (CCW) until I could see the vacuum start to drop off. This setting seemed to get me right to the oil cap off test.
the oil cap off test will be when it idles well but as soon as you remove the oil cap it will start to miss and quickly die. This is due to unmetered air causing a over lean condition. If you remove the cap and nothing happens you are a bit rich at idle but will look the best on the vacuum gauge. if you remove the oil cap and it speeds up you are too rich to the point where the engine is no longer at optimal mix even for best vacuum. Again I use the vacuum reading as a way to determine if the engine is operating at peak efficiency.