So trying to answer my own question:
The fuel rail pressure is a constant 4bar, this never changes. So the differential across the injector would be 4bar which is 58.02psi - gauge pressure in the manifold which would be MAP - Barometric.
The higher the differential the more fuel will flow and the lower the differential the less will flow.
The "Flow rate low/high mult vs pressure" table tells the ecu how much more or less fuel will flow at a given pressure differential.
So, I should be able to dial in the multipliers by creating a math parameter to calculate the pressure differential and logging it against my fuel trims using the same x-axis as the multiplier table.
Then I can adjust the multipliers (somewhat counter intuitively) up to lean and down to enrich.
Anyone see why this wouldn't work?