I compared the injector pulse width versus the ECU pulse width across a range of speeds and throttle settings to get an average ratio between the two. Here's the graph of the relationship:
The Raw data is for the full injector pulse width and the -500uS data is for the inj pulse minus the settling time. So, the ECU pulse width seems to be about 1/8th the width of the injector pulse and it's consistent across different speeds and acceleration.
If I assume that the ratio is 0.123 and I know that the injector uS/gal is 200500000 for my car, when I switch to the ECU signal input, the uS/gal should be 148000000. Calculated as 0.123 * 200500000 * 6 inj pulses. That gives me a starting point to calibrate the ECU signal.
Of course, it still remains to figure out how to handle the ECU flatline signal when coasting.