72 or so hours in, and I'm up to 0.7.
So I'm definitely gonna say the meter is fine. Just took me a bit longer to bump up to that level during this fast. Might be that the difference last time is that I was doing a fat-supplemented fast. So perhaps I was taking advantage of some of that fat input to make up for what I couldn't provide from my own supply.
Thanks again, @yogipete, for starting this thread. While it may not be as precise as a blood ketone meter, the breathalyzer at least provides some useful data, and it's a heck of a lot cheaper.