"What is the official pagan symbol people draw whilst casting a spell in the 'pentatonic' category?"
For there to be an official pagan symbol there would need to be some understanding of what is and is not officially pagan. There isn't. Paganism is a blanket term for all religions typically thought of as being indigenous, polytheistic, and usually rather primitive (hence the reason Hinduism isn't typically included).
And pentagrams as used in magick generally have absolutely nothing to do with religious symbols. The LBRP uses pentagrams for their elemental symbolism and their sacred geometry, not because pentagrams are pagan.