Performance Measure with PCP score

Need to calculate the PCP score will be computed for the lower arms, upper arms, lower legs and upper legs. The total score will be the average of the arms and legs PCP score.

The PCP score of a limb, defined by a pair of joints, is considered correct if the distance between both estimated joint locations and true limb joint locations is at most 50% of the length of the ground-truth limb.

Can someone please explain the concept of calculating distance estimation?