Check the voltage before you even loosen those screws. If it's .98 to .999 then leave it alone. TPS is only an issue if it set way too high or way to low. This idea of getting it set at .999 is worthless. The most important thing is that the voltage goes up to 4.6-4.7 at WOT and that it does it in a linear fashion. No drop offs, spikes, flat spots.
Oh, your ignition has to be in the run position. Just don't start the car.
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351W 89 Mustang GT Convertible