I'm a 37 botanist, but I imagine it isn't much different for miner. I've been farming the 4 node eval leves. Use sharp Vision/Field Mastery III skill for 50% bonus and get the 9 eval items 100% of the time on two of the nodes and increase difficulty rating by 1 or 2 levels to get the unbuffed gather percentage around 50%. Get all 7 eval items on the other 2 and if you can use the ability to increase gathering rate to 100% after chain 4 then go for the 9 eval. I'm able to get the +25% bonus every time.