I, like most people I assume, read my digest from top to bottom. My digest can be pretty long on some days, and it's easy to lose track of the "mark as read" button for the day. I have to scroll to the top and click the button - but then I lose my place, because I sometimes scan multiple days.
I think if the button were included at the bottom of each digest, preferably in an always visible way justified to the left, I would make more and better use of Throttle.
It would be nice if days were more clearly separated too, perhaps with a full page line - so that I don't skip multiple days without marking them as read. Otherwise my daily digests just start to feel like a spam box - TONS of things to review, no desire to actually do it.