Dropping vowels

I wondered about the dropping of minor vowels in Simplified (although I'm sure this applies to earlier Gregg). The word "picture" is a good example of this, being written as p-k-t-r.

I'd tend to include the first vowel if I didn't know it wasn't used in the standard outline. There are other words I sometimes write during a lesson where I'd omit a vowel but see that the lesson outline uses it. Is there a good rule of thumb for this, or is it like the left vs right "s" where I'll develop a feel for it with experience?

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