How about 200 miles?
The formula for range based on elevation is simple high school geometry:
R-squared (the radius of the earth) plus r-squared (the range) equals quantity (R+H)-squared (where "H" is the height above the earth. Solving for r (the range):
range = square-root of (2HR + H-squared)
Since normally "H" is insignificant compared to 2R, we can ignore the "H-squared" term.
So range = square-root of (2HR)
All variables are in miles, and in case you've forgotten, R (the radius of the earth) is about 4000 miles.
So, for an elevation of two miles, the range is about 125 miles.
If you want to listen to airline traffic, that's usually up there about 7 miles, and for that the range is about 235 miles.
Of course, if you happen to be in another airplane at (say) 10,000 feet, then the range to hear airliners (all other things being equal, which they are not) is the sum of the two examples above, or about 360 miles.
I say all this because as a pilot, I think the 50-mile limit is WOEFULLY inadequate. A 200 mile limit would be adequate for ground-based stations.