Defi Boost Guage question
 

I just finished installing my guage. Outside from my dimmer switch problem, I'm having trouble getting the guage to show the correct reading. If it is correct, then I'm making good hp off of -20 to 0 boost. The sensor is mounted on the passenger side to the body with a stainless steel bolt. I connected the sensor to the extra nipple of the manifold. The wire runs along the top of the firewall and enters the cabin via the steering wheel boot. I used one of the steering wheel mount bolts near the firewall for a ground. Any ideas as to the problem?