I was testing xd, dy and dz and I figured out that I need to -1 from dx and add 2 to dz and dy doesn't work. I hope you can fix these porblems!
Comments 3
Choose the corner coordinates with the lowest x, y, z values, then offset dx, dy, dz from that. If you hit F3, the little orientation thingy in the middle of the screen can help you, the three 'legs' show in the positive direction for each, x, y, z, so you find the block furthest away from each positive direction within your test area.
dx, dy and dz cannot be negative.