From the tutorial on the website, breath acetone correlates well to blood sugar levels. Therefore seems to be highest after fasting. My level are a little lower after eating meals and get higher hours after a meal.
In general i have found that i discard the first attempt as its normally a low number (i don’t think i get the last of the breath out on the first try) but attempt 2 and 3 are usually higher and more consistent.
I only blow out the last quarter of my breath into the sensor gently and i push the last of the breath out. This way i don’t feel like I’m going to pass out.
Readings are altered for about 18 hours post alcohol consumption so i wait until after lunchtime if I had a drink the night before.
Food alters readings for about 45 minutes so i wait until then.
Its a good device once you get used to using it correctly get your technique worked out. Keep persisting until you get consistent readings!