I've been very lucky with my mining. I mine up a stack of each material while leveling and then check how many HQs I received. It is always above the 15%, and usually even at or above the 25% I can occasionally buff to.

99 alumite / 33 HQ alumite = 25%
99 iron ore / 27 HQ iron ore = 21%

I also don't get very many bad streaks in the gather percent. If it is at 70%+ I don't remember ever failing twice in a row. Sometimes in the 60% range though I'll fail a whole node. I'm happy if I'm trading some luck on gather chance for higher yields on HQ items!