I'd count the letters to find the expected area, then get the radius. (Keep in mind the aspect ratio of your character cells...ellipses (pun intended) might be better to work with.

Then use your trig relationships to compute the "best" number of characters in each line. You might then use a recursive backtracking algorithm with a simple heuristic to fit the words to the lines as good as possible.

That's how I'd start it, were I to do it.

...Roboticus