I'd put the gauge between the solonoid and the nozzle so you can see a momentary drop when the valve opens. Gives you some positive feedback that the system is actually functional. Would also show if a line was disconnected or broken anywhere between the pump and nozzle.
After the shutoff solonoid. Let the gauge tell you you're delivering.
That sort of makes sense but when the valve is closed, won't it see vacuum and when it opens it will see pressure. There would also be more fluid to suck out and have to be refilled. Only if it'* before the solenoid, will it see a momentary drop.
It it is before the valve and the valve doesn't open, it would not know, so putting it after would make sense.
Since I am going to leave the gauge in the engine bay, I won't be able to monitor while driving anyway.