[ix101]

I would argue that it's harder for me to decide how easily I recalled a word and decide between a few loosely defined levels as to which one I should choose than to apply a simple algorithm.

If the window loses focus it would be able to pause automatically. If you are distracted another way, no big deal you will see that word again soon and unlikely to keep getting distracted on the same word. The benefits would outweigh the odd misfire.