10% from left = Like the LEFT person very much more
20% from left = Like the LEFT person much more
30% from left = Like the LEFT person more
40% from left = Like the LEFT person slightly more
50% from left = Like both people EQUALLY (middle of line)
60% from left = Like the RIGHT person slightly more
70% from left = Like the RIGHT person more
80% from left = Like the RIGHT person much more
90% from left = Like the RIGHT person very much more
So, the more you like the person on the Right, compared to the one on the left, the farther your mark will be to the right. If you would like or dislike them equally, you should click in the middle of the scale. If you like the left one better, your click will be left of the middle, and the more you like the left better, the farther to the left your mark should be.
REMEMBER: You are judging the DIFFERENCE between TWO people.