That is a very good explanation of the basis for your circuit.
Another design issue: While your car is running, the voltage regulator in the charging system will keep the battery voltage between 14.0V to 14.5V. A simple voltage divider can reduce this to the reference voltage against which the TPS signal will be compared. However, that reference voltage would have the same percentage fluctuation as the battery voltage, i.e. when the battery goes from 14.0 to 14.5, the reference voltage would go from 4.00V to 4.14V, so if that is compared to the TPS voltage, there would be a slight shift in the trip point. Is that enough of a shift to warrant regulating the reference voltage?