A baseline is your ketosis "a typical day". By creating a baseline you can then make lifestyle changes and then compare the effects of the change against the baseline.
An example: You want to lose weight (fat-mass), but you hit a stall (even you are eating ketogenic, your weight is stabil, even increasing!).
Step 1: Measure every day for a week whenever is suitable for you. Once an hour or once every X hour. When your week is done, you will have a good sense of how your "normal" ketosis looks like.
Step 2: Now decide what type of lifestyle-change you want to try ... exercise? diet? fasting?
Step 3: Stick to your "lifestyle change" and measure like you did when creating your baseline. The measures should generally be higher than your baseline if you succeeded to increase your ketosis.
Try to measure a whole week. This captures effects of different types of exercise, some triggers gluconeogenesis to acute replenish glycogen stores and have a ketogenesis effect that kicks in hours later. Exercise in the afternoon/evening have a effect on your morning ketosis ... etc.
Example of baseline and after a "lifestyle change". The image to the left is the "baseline" day, the image to the right is a "after lifestyle change day".
When looking at the diagrams it is obvious that the lifestyle used in the right context produce more ketones than the previous lifestyle.