There is no consistent rule, unfortunately, other than the omission of minor vowel rules as noted in paragraphs 50 and 88 in the 1916 New and Revised Edition (paragraphs 16 and 121 in the Anniversary manual). In general, the schwa sound vowel will be written in shorthand using the orthographic vowel if the word starts with the schwa sound. In the middle of the word, it is trickier, as sometimes these vowels are omitted (omission of minor vowel rules). Even there are differences in Gregg series: for example, in Simplified, the e in the word camera is omitted. Although there is no consistent rule, if you read a lot of shorthand, this becomes second nature, and you will know when to write the vowel and which one to put it. Further, whether you insert a vowel or not should not make a difference in your own writing.