Set a minimum quality level for each cave level. +10 base 1 for each level. So the lowest you can find on level 5 would be q50. Not this Q20 black ore on my level 5. We can't track down good quality nodes now due to the change in stone and ore quality, so you just have to hope you don't get shafted every time you build a minehole.
Mining isn't fun right now. Something else needs to be in place if this is how quality for stones is going to continue working. We need a way to prospect and get a hint at quality, or have a minimum quality. I'm still using the q60 iron ocre from level 1 because its the best shit I've found on any level, multiple mineholes to each level as well.