Using card as a measure?

Hi everyone.  I am VERY new to PBI so I may be stating some things incorrectly.  I am making a part usage report for our warehouse and I am trying to calculate the per day usage of each part.  So if I slice the report for 30 days, and use 60 of a single part, the value would be 2.  I am having trouble understanding how and where to capture that date.  I have a card displaying the number of distinct days being sliced.  A collegue told me to chack out using a variable but I am not ubderstanding that part just yet.  I think once I can get that value somewhere I can do the rest of the calculations.

Thanks

Chris