there's a doc on their site explaining how to tune it in detail (called something like "tuning tutorial" from what i recall). You'll have to max the MAP output at your max boost, to start with you can use blend feature between min vacuum and max boost and level off at that (read the manual, it is explained there how to get started), other than that use the cells to modify the output voltage based on your AFR at that cell (vac/boost & rpm) make small adjustments unless you're way off on the AFR. You use the cells to modify the volts out, the cells are at a point intersecting vac/boost with rpm (this is to answer your question as in what to modify), so to conclude the MAP voltage output to PCM is based on vac/boost and rpm reading and the cell value.