Great question. I used to use the “no symptoms below the chest” rule, but I was also worried that it may be too conservative. (Note: too conservative is better than not conservative enough.)
Now, when my symptoms have improved and my energy level is back to normal, I’ll do a very easy, very short run or walk (20′-30′), and then see how I feel. If I feel fine, then the next day maybe another. If I feel worse, then I back off again. In general, I look at the trend: am I improving or regressing? And then adjust accordingly.
Again though, it’s far better to underestimate your recovery status than overestimate it.