I'm surprised your numbers are that low on those transponders. You would likely be okay most of the time, but you would probably lose locals pretty quickly in a moderate rain.
The signal strength for the Chicago locals was near 100 on each transponder, St. Louis locals were in the mid to high 80's on each transponder. It seems crazy to me that the Springfield market is so low as far as signal strength. I am only 60 miles from Springfield and 90 miles from Champaign, well within range of the spot beam.