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I don't know how it's done on a Geo, but most fuel injection ECUs compute the engine "load" and above 80% load for a given RPM it will enter open loop.
At very low engine speeds engines with large-ish throttle bodies can exceed 80% load with very little throttle input, but at high speeds such load may require full WOT or nearly full.
I doubt the TPS is the sole factor determining when to cross the open loop threshold. A speed-density ECU will use a MAP sensor to determine load and when the pressure in the intake manifold begins to near atmospheric (less vacuum) it will go open-loop. Other vehicles will use a MAF sensor and when a computed value of airflow is exceeded at a given RPM the ECU will know that the load is high and will enter open-loop. Both systems might have an additional requirement that a minimum throttle position is exceeded in addition to the above requirements, but that may or may not even be there or it may vary depending on engine speed.
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