If you were prepared to put the pressure sender at the rear with the the pump outlet, then you could measure any pressure drop between there and where the fuel pressure sensor currently is.

And by moving the sensor around you can measure the pressures and work out exactly where the pressure drop is in the system.
EG :
After the rail but before the reg,
Before the rails,
Before the filter,
Before the hardlines,

But this is alot of mucking around. It would be just as easy to replace fuel rails and hosing at the engine. The standard hardlines are capable of way more than 300hp of e85.

Are you using one of those hardline adapters to fit the different puel pressure regs?

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Doing it for all the wagons out there