Quantity and Date Range Calculations

I am working on a formula that calculates the number of milligrams of aspirin that a patient consumes daily. A patient consumes varying amounts of aspirin daily. A blood test determines the concentration of aspirin in the patient. The test results are always fluctuating.

Here is some sample data:
Patient takes the following quantity of aspirin from January 1, 2018 to January 3, 2018: 1-1-18: 1 mg of aspirin;
1-2-18: 2 mg and 1-3-18: 3 mg.

Calculation: 6 divided by 3 = 2 mg in 3 day period. Result is 2 mg per day.

Here is the formula that I am trying to work with:
sum({Aspirin_Dosage.Dosage},{Test_Date.TestDate},”1/01/2018, 1/02/2018, 1/02/2018”)/count({Aspirin_Dosage.
Dosage},{Test_Date.TestDate}, ”1/01/2018, 1/02/2018, 1/02/2018”)

Database fields (Access):
{Test_Date.TestDate} = Need 3 days.
{Aspirin_Dosage.Dosage} = Need to add daily dose in mg for 3 days.

Report = The attached report is grouped on the {Test_Date.TestDate} field and “for each day” is selected under “the section will be printed” option under “Options”.

Screenshots of the database tables are shown below. How do I write a formula that performs the calculations?
 Daily_Dosage2.rpt