Sorry for the stupid mistake before.
I totally forgot about only using the real-air part (height) like checkerboard said.
Using 4% for 10 degrees deviation from isa you get: 34 degrees warmer than standard on 1200 feet height, This makes true altitude about 14 % higher than indicated. So 14% of 1200 feet is 168 feet.
This is close to Checkerboard's answer so I hope it is correct now.