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When I learned astrology, I went the usual contemporary astrology route. I followed the rules even if the rules changed from one astrologer to the next. I continued to have just enough astrological epiphanies that I stayed interested and motivated, but I was not happy with the astrological advice that I was getting. I could see that many of the standard methods weren't working nearly well enough for me. The dispositor method was particularly unreliable.
I found a book (I was buying many books) called "Synthesis" by Tracy Marks. That book kicked me in the butt, and my accuracy and skill jumped up a half dozen notches. It opened my eyes, and I began to explore further. Now, I don't use her method today because I have refined my own, but her method helped me in my path.
Most people declare the chart ruler by the planet ruling the Ascendant unless there is something conjunct the Ascendant, then it becomes the ruler. What I say is, there is a better way, and you will find NOT just one ruler but a pecking order of rulers that leads you through synthesis.
What planets are closest to the four angles, As, Mc, Ds, Ic? Those are the rulers. Often the closest is the chief. You may find aspects this way, and it is great if you discover that there is a ruling aspect rather than a ruling planet because an aspect is more focused and specific.
For Buckingham, It is Mars/Pluto/Venus. Rock (Mars) Music (Venus). The Mars/Venus square is the ruler of his chart. It is more reliable than the sign on the Mc or most of the other guides that you are given.