I'm surprised the angle of the MAF makes a difference, there must be a temperature or pressure difference in the intake. It could be that part of the MAF is dirty, so the amount of airflow over the cleaner part varies depending on the angle.
Usually the distance from the engine/turbo is important as they both generate a pulsing (either by the intake stroke of the pistons or by the turbo itself, which likes to be driven by pulsing from the exhaust stroke). The pulsing can give false MAF readings but this usually settles down the further away from the turbo you get, although too far and you get poor throttle response as the airflow readings aren't quite true.