Tire Pressures

I took my Swinger to the track last week for some 1/8 miles passes with a new set of 27-10.50 15 Hoosier QT Pros (tubes, no rim screws). I started at 18lbs and worked down to 15lbs. My best 60 was 1.86 with 15lbs but didn't go any lower. I haven't been able to find much information on recommended Hoosier tire pressures. This is a 3200lb street car, frame connectors, S/S springs, snubber and runs low 8.30s (1/8 ) and high 12s (1/4). I know I can get more out of the tires and the car. I also know most of it is trial and error (and track and air conditions ) but, how low can I go on tire pressures? Suggestions?