The affix ṭac comes after sakthi when preceded by 1. uttara 2. mṛga 3. pūrva in a tat-puruṣa samāsa as well as when a thing is compared with it.,
By the force of the word च in the sutra, we read in the word उपमानात् also. Thus उत्तरसक्थम्, मृगसक्थम्, पूर्वसक्थम् ॥ So also in फलकमिव सक्थि = फलकसक्थम् ॥,