I was thinking about your fuel volume control using an additional spring loaded plunger set in the side of the injection pump barrel. Without the additional plunger as soon as the solenoid is energized the main plunger will rise displacing fuel and causing the inlet valve to close. The pressure will then build up (rapidly as the fuel is almost incompressible) until the delivery valve and injector open. Fuel will then be delivered at a rate determined by the force of the solenoid and the inertia of the main plunger and fuel.
If you now incorporate the side plunger the main plunger will have to displace enough fuel to move the side plunger to the limit screw before pressure will build up to allow injection to commence. With a cam driven system the delay could be readily calculated and would be the same as if a wedge system was used. With the solenoid driven this would depend on the force of the solenoid and the inertia of the main and side pistons.
This may be just a theoretical issue and me overthinking.
