Made a little progress, built the circuit (with reverse diode instead of zener) and tried it out on the bench. The standard guino would be showing about 0.53 gph but this is showing 0.77, so there may be some futzing yet as the opto may add delays and possibly innacuracies, but should be better than nothing.
There is an additional lead that should be connected to the plus side of the injector to eliminate variables.
Note: If I change inj delayUS to 1600 then it goes back to 0.53 gph, don't know if that is the correct course of action yet, just sayin.