Nice guide. Can I add something, but if it's already said by someone else other than you, then please discard it.
If you're not a sense-type, try to search for places where other people wouldn't drill too much, for example, in in our very own ruins, drill in a room that has a curved or bent opening, the lesser the people that drill there, the faster it becomes a hotspot.
Believe it or not, items do SPAWN at the same place. If you've drilled a really RARE item there, wait for around a max of 5 minutes and drill there again within a 20x20 grid. For example, the Bloody Runes of Fate, I just drilled them in one place :D and I got 2 in a row. I had a total of 21 from an hour of digging.
There is no such thing as a best or worst spot to drill in a map, but there really are respawn points (the difference between this and a hotspot is only minimal, it's just that when you dig in that area and come back after a few seconds, you'll be amazed that the area you wiped out was completely filled again with items to drill.) This is a place where items really respawn FAST. :D
When I get back into playing into Trickster... maybe, I'll be back to power-grinding only. College really is taking quite a lot from me. :D