I wanted to know if it's possible to use customized formulas in Tables:

For exemple: 

I have 100 customer at day1 who enter my shop and 13 customer who actual buy something. So I have a "Buy-rate" of 13%.

My database resumes these datas day by day.

I wanted to show a Total of the data (in a pivot table) of my database but my "Buy-rate" isn't correct. 

So I want to add a formula(Customer who buy/Customer who enter the shop), like I do right now in my Database, to bypass the wrong "TOTAL" caused by the average formula.

I hope my explanation is clear and that you can help me with this problem.

THANKS