> Normally you would pick a font size or line length, and find a fitting lineheight last.

Agreed. In my experience building for the web, line length is usually the constraint that other dimensions are chosen around: the width of the device, a fixed-width column of content, or a specified width for a dialog or modal.

In this game, I was able to easily score perfect for line height and font size sliders, but it found it quite difficult to score anything above 0 for the line length slider.