For decades, Calligraphy has been a popular artistic activity in Japan, China, and some countries. To assist its self-learning, we have proposed the Calligraphy Learning Assistant System (CLAS) using projection mapping, where a learner can practice it by following the letter writing video of a teacher projected on the paper. In this paper, we newly implement a letter portion practice function in CLAS, such that learners may practice their weak portions with the video showing the writing by a teacher. It is expected that to repeat practicing weak portions is useful in improving the whole letter writing. Through applications to 12 novice students from Indonesia, Myanmar, China, and Kenya, we confirm the effectiveness of this function, where each student has significantly improved the calligraphy skill.