Do I take it, then, that the highest title is the one to go by, even if you're writing the individual in question in their capacity as chief of one's clan?
I think that as a general rule of thumb, you are correct - go for the highest title. There are exceptions. One being the military, where the rank comes first - Captain the Lord Teacup or Major Doctor Jones.

There is also a thing called Courtesy Titles. If a man is the Earl of Highchair, he might very well also hold a lower title (Baron Footstool) which is loaned to his eldest son during the father's lifetime by courtesy. So if we address the Baron Footstool, Chief of Clan Heartrug, we will be addressing the Chief's son and not the Chief.

This works in all walks of life. When I was growing up, I learned who I had to call Sir and who I could call Mr Jones. Who I could call George and who I could call Oi You Idiot. This has stood me in good stead throughout my life.

Regards

Chas