Oh I see! Why didn't I think of that
Good news.
My breadboard is looking dangerous so I'm taking time out for an enclosure, decent connections and tidying up. This is my daily driver so I can't afford any mistakes. Besides my partner thinks I've lost the plot, what with filling the car with bare wires!!
I got as far as multiplying pressure by a thousand for the mpg calc, and at that stage mpg was kind of making sense. I get the impression that pressure has a far bigger effect on fuel burnt than injector duration.
I have read that 1350 bar is unlikely to be seen- since there is only a small window at 4000 rpm where it's called for. And in any case my pump is 120 000 miles old!!