My rule of thumb depends on what I'm planning to ride. Because I ride all
manner of surfaces, I have 700 x 40 tires. On asphalt I run about 80-85,
which with a strong thumb compresses 1/4" to 3/8". On the trail I let some
out, not having suspension, to 65 or 70, or 1/2". This excess gas usually
winds up in really old threads.