The aim of this paper is to suggest an index of coarticulation, proportional undershoot values, given the observation that absolute undershoot within a language tends to be proportional to target-locus difference. The target-locus proportionality predicts that a large difference between the consonant locus and the vowel target will result in a large amount of vowel undershoot, while a small difference a small amount of vowel undershoot. It turns out that the proportional undershoot is a potentially more appropriate way of comparing degree of undershoot across languages. However, even though the proportional undershoot measurement may provide a useful index comparing the overall coarticulation degree in a CV token for cross-linguistic data, it is concluded that it may potentially wrongly predict the cases of transfer or error as a progress in learning.