Well, it's been a couple days.

I've been out of town, so my wife has just been testing and adding to 16ppm each night.

so:
7-2 11pm - wife brought it to 16ppm
7-3 11:30pm - at 7ppm, shocked to 16.6ppm
7-4 9:00am - 14ppm

So, the loss from 9.5 hours at night was 2.6ppm.