Oh, a gauge, ok
Can you confirm the voltage range of the pressure sensor @ 1350, and the correlation of voltage to displayed pressure?
It is just an analogRead to convert a 5 volt signal to a value from 0 to 1023. Recommend Analog Pin 2.
At the top of the loop, for a quick test anyway, you can do an
int press = analogRead(2) + 1;
then multiply the instant.injectorHiUS by press after copying it from temp but before updating the other trips.
Fudge microSecondsPerGallon to taste.
press *might* need an offset and a scaling factor, don't know yet.