Hmmm. That's strange. I wouldn't have called what you linked "the exterior angle of a regular heptagon". I'm not sure what I would call it exactly.

But it is clear that the value you're looking for is related to exterior and interior angle by a simple formula:

(angle you're looking for) + (interior angle) + (angle you're looking for) = (exterior angle)

I probably would have called the angle you're looking something like the "tangent angle", which seems to be a term used by others as well. A little of bit geometry also shows that the angle you're looking for is really just the "central angle" of the polygon, which you can get by searching "central angle of a regular heptagon".