If your reading is not accurate then it could be two things. First the steel rod that goes from the transmission to the steering column is out of adjustment. Just loosen up and re adjust at the trans linkage as needed. Second the wire that attaches to the column from the indicator has a small clip and is out of position. You have to remove the dash bezel to see it. It's on the lower drivers side and can be slide around the circumference of the column for corrections. As long as the shifter cable isn't stretched you should be ok.
1986 Monte Carlo SS LS3 build almost running. NOT.....LOL
Sold all my cars but the Super Slug. I will rebuild the car and up the bar a bit. Ridetech, custom frame, new supercharged LT4 or a LSX 454 and much more.