Experimental evidence suggests that people underestimate relative depth for distractors beyond the fixate () and overestimate closeness (or ) for distractors nearer than the fixate. The motion/pursuit ratio has this relationship to the motion/pursuit depth formula, so it is possible that the ratio alone is a better estimate of the perceived depth.
The Demonstration "Motion/Pursuit Ratio and Depth in 2D (Visual Depth Perception 18)" (see Related Links) compares the motion/pursuit ratio and the motion pursuit law in two distractor dimensions.