I am collecting COVID-19 statistical data for my clinic.  I want to calculate and display the positivity rate over the past 10 days on my data entry form and on my printed reports. Moreover, I want this calculation to update daily.  

The calculation:
(Positive) / (Positive + Negative) tests.  
Plainly stated:
o  The number of positive tests is divided by the sum of all positive and negative tests.  

My database currently uses a combo box to collect the following values:
Positive
Negative
Pending
Unknown  

I feel like the solution is just out of my grasp but I am all out of ideas.  HELP!  If you find that to be too easy, perhaps you can tackle the 7-Day moving average :-) .  
Thank you,
    Windle