Indeed, Rudi—the upsweep towards less-than-none exposure looked humorous. Bear in mind, however, if I you increase the number of degrees of freedom on the Bezier spline to 7 or more, the toe looks just fine, yet the calculation still gets the CI—what we miss then is the smoothing that neatly takes care of the statistical measurement error. Having said that, I have to play, yet, with other types of splines, including log-influenced ones, which might have that "perfect" look.
But the next thing I plan to do is what Bill has suggested, that is the Time/CI curve. By the way, if anyone has a curve data series with their own, known CI, I'd be happy to check the algorithm against those numbers.