Good Condition Probability

This may be a well documented answer, but I looked around briefly and could not find it. Does anyone know, off the top of their head, what the probability of a Good condition taking place is?

My own estimation based entirely on empirical evidence puts it somewhere between 1/3 and 1/5, but I haven't actively recorded the incidence rate of Good quality, so I could be off by a huge amount.

I did a small sample a few weeks ago. Out of 100 orb changes, 19 were good condition. I know this is a very small sample, but around 20% seems about right overall.