Could you check what voltage level is on MAP signal when you have -0,8bar or 1,5bar? I think these values from datasheet may be not correct. And do you mean -0,8bar where reference point is atmospheric pressure? Which value was atmospheric pressure, 1,0 bar or lower? We need absolute pressure for scaling, not relative.
You cna also check your readings from ECU and scale using these values, e.g. when your ecu shows 48kpa (absolute) what voltage do you have on MAP signal? You will also need second value for scaling (sensor is linear)