Ela, I dont think so, here it is very straightforward code, DB-> UserProfiles-> reading fram.
Here, in your code it seems to be one user will be owner of the db or one user control. And you are seems to be done Great, as you have sucessfully save the namespace for one day and under that namespace you have saved the one reading of pulse oximeter sensor for different fingers ( I just visually verified ).
But in the ABG's code example there was a logic to have namespaces for profil(s), but here you are just saving multiple readings for one day, and perhaps for one user only. Why don't you have a dictionary, day as Key and list of lists as value for multiple readings. Your app will be having two options in main page one for having new reaidng and one options of read old readings.
Go to a sepcific namespace and populate the underling tag using get all tags and enumarate them all to render them.
The way you had saved the data, the same way you will be using to retrive them, millis or a specific date-format.
and maximum querey/concerns have already been answered here