Natyashastra
गुरुप्रायाऽवकृष्टा स्याल्लघुप्राया द्रुता तथा । गुरुलघ्वक्षरप्रायाः शेषाः कार्या ध्रुवास्तथा ॥ ४०॥
The Apakṛṣṭā Dhruvā should consist predominantly of long syllables, and the Drutā Dhruvā predominantly of short syllables. The remaining Dhruvās should be composed mostly of both long and short syllables.
english translation
guruprAyA'vakRSTA syAllaghuprAyA drutA tathA । gurulaghvakSaraprAyAH zeSAH kAryA dhruvAstathA ॥ 40॥
hk transliteration by Sanscript