I tried a bunch of tweaks to the formula that calculates the positions of the 20 vertices comprising the circle. Unfortunately, all of them are off enough that the circle is still oblong, and it seriously morphs depending on where on the globe the central point is. The formula that I ended up working was:
Length of a degree of longitude = cos (latitude) * 111.325 kilometers
from this source.
So my conclusion is that this formula must not be accurate enough. I spent most of the rest of the day yesterday searching various google earth forums, and happened on this post. It seems to calculate the vertices using the haversine formula and great circle distances. I think this is the next method to explore.
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