@dfabulich, I was wondering if negative achievements (i.e. achievements that reduce your score) are possible?
I tried it, and it seemed to work, though I think I found a bug.
I have 4 achievements, two of which I gave a negative value. Testing them, I achieved the first one, and when I went to the achievement list, it reported that I’d achieved all 4, getting a perfect score. This is the bug. Once I achieved the second negative achievement, however, it behaved properly, reporting only 2 achievements having been achieved.