Agni Purana 24.13

24.13

khātasya mekhalāṃ ramyāṃ śeṣārdhena tu kārayet /
kaṇṭhaṃ tribhāgavistāraṃ aṅguṣṭhakasamāyataṃ //

With the remaining half, one should make a charming belt for the excavated hollow. / The neck is one-third in width and equal in length to a thumb;