Airmass per intake stroke = (MAP - offset)/slope
LOAD = ((MAP - offset)/slope)/displacement
This is in the simplest form and does not take into account slope and/or VE correction. The actual algorithm is much more complicated.
If your afr is lean at the same MAP then you need to lower your slope. If your afr is correct at one MAP value and then lean at a higher MAP value then your offset is out.
The offset has a huge effect at low MAP values.
Remember that the airmass calc is used in the load calc so your spark advance will be affected as well as many other things.
Also it is essential that you start with the correct injector scaling, something that can't be done with the nizpro's etc. Most tuners simply 'touch up' the SD tables to smooth the afr's.
A full SD tue is very demanding on the vehicle, not too mention time consuming given the unique slope and offset for each cam angle.