You can't directly use a MAP sensor reading to calculate load
Not unless you can correlate the manifold pressure to a flow rate value.
elmerfud's answerelmerfud's answer shows an equation involving volumetric efficiency (VE), which essentially represents that correlation between flow rate and pressure. This correlation will vary between engines and the only way to obtain it is through experimental testing.
An old-school way to establish the relationship between pressure and flow rate is to install a flow meter in the duct work that provides flow rate measurements, which can then be used in conjunction with the air pressure measurements to establish the relationship.