Anderson, Francis I. and David Noel Freedman, “The Orthography of the Aramaic Portion of the Tell Fekherye Bilingual” in Text and Context: Old Testament and Semitic Studies for F. C. Fensham, ed. W. Claassen, JSOT Supplements 48 (Sheffield, 1988): 9-49.
Anderson and Freedman are mainly interested in the spelling practices used in the Aramaic portion of the inscription. Their method is to examine the inscription by analyzing all of the words in which vowel letters potentially occur. They conclude that the use of vowel letters is consistent with their analysis of Northwest Semitic inscriptions from Judah, Israel, Ammon and Moab. All final vowels are marked by yod, he, or waw. Aleph never occurs as a vowel letter, it is either a consonant or a determinative marker. Both waw and yod are used to mark medial vowel letters. The scribe tends to limit himself to using one medial vowel letter per word. The use of yod for /i/ seems to have its basis in derivation but the use of waw for /u/ may be analogical and purely phonetic. This would suggest that waw for /u/ could be used more freely as there would be no confusion with historical spelling. For the most part, medial vowel letters tend to mark long vowels in stressed syllables.