Mayamatam 19.22

19.22

bhittivyāse dvādaśāṃśe tubāhye pañcāṃśāntadvārayogasya madhyam
tadvaccāntaḥ stambhamadhyaṃ tayostanmadhyaṃ proktaṃ bhittimadhyaṃ vidhijñaiḥ

When the wall's width is divided into twelve parts, the center of the doorway frame is on the outside at five parts; / likewise inside is the center of the pillar. The middle between them is called the center of the wall by those who know the rules.