Unless you have a turbonormalized or turbocharged aircraft, your manifold pressure can never be higher than ambient pressure, only lower. You regulate this by closing and opening the throttle valve.
The amount of fuel that's sucked into the cylinders is depending mostly on the amount of air that's sucked in (through the carbs). Less manifold pressure means less air is sucked into the cylinders, and so less fuel is sucked in with it. (Yes, it also depends on the mixture settings, but we'll leave that out for now.)
So it doesn't matter at all what the exact throttle position is: fully open to the stops or just wide enough open to equalize ambient with manifold doesn't matter one bit. You can even see this at ground level: the last centimeter or so of throttle movement, when you open the throttle for takeoff, doesn't matter in the RPMs, MAP or fuel flow at all.