Quality is +1/2/3.

To avoid certain items the best is to follow the tips they give and move the depth gauge the opposite way. Some examples are iron ore and those raw jewels...the depletion they cuse is terrible grind-wise so the best would be, since they often are at deeper areas, to move gauge up so you might have them much rarier.

Also, being in a spot where you get there "you'sense the [item] right where you are" of a common item would maximize the chances of obtaining it...having iron sand, pyrite or copper ore in this situation helps alot.

Also, its nice that the item dificulty is the very same no matter the grade...if you mine a pyrite in glory area (coerthas, grade 4 if not mistaken), it will deplete your remainder there just as fast as it would if you mine it in black brush area (thanalan, grade 1)...so seeking low rank items is the best way to improve your sp/hr if you don't care about drops.