The two numbers you need to read a compressor map: how hard the compressor has to squeeze (pressure ratio) and how much air it has to move (lb/min) for your horsepower goal.
Pressure ratio compares the air leaving the compressor to the air entering it. Any restriction in the air filter and inlet lowers the entering pressure, which is why it's subtracted. Airflow comes from how much fuel the engine burns (horsepower × BSFC) multiplied by how much air goes with each pound of fuel.
Find where your pressure ratio and airflow meet on the map. You want that point inside the efficient islands and away from the surge line on the left.
Method follows Garrett's published guidance on reading compressor maps.