So I bought a refractometer...
 

Now I'm trying to nail down what I'm reading. Mine is temp compensating, and I calibrated it with the distilled water and unit being in the same environment. Tested my new coolant (Porsche) and got a reading of -30 deg Celsius. The 50/50 chart on the bottle states protection level @ -37 deg Celsius. Given mine is reading -30 should I add a bit more undiluted coolant when it needs it, or just leave it as it is and be good to go?