Okay, you were kind of doing two procedures at once. I suppose that's okay. Unless you have digital tach, or one that's really accurate in the car, the best way is to use the feeler guage method. This is the one that the service manual says to use. The method they used on corral.net is actually for the 6-cyl.
Back the throttle adjustment screw out until you can fit a .010" feeler gauge between the screw and the stop plate. Turn it in until you can feel a little resistance as you pull the feeler gauge out. Turn the adjustment scew another 1 1/2 turns. Disconnect only the negative battery cable for about 15 minutes. Put the cable back on and start the car. Idle with no accessories for 10 minutes. Turn off. Start. Idle with all accessories for another 10.
See how it runs then. If the car runs well with the IAB disconnected, that probably means the idle is set too high by the adjustment screw.
If this doesn't work, there probably is something wrong with the IAB.
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351W 89 Mustang GT Convertible