Calculate rate from two tables

Hi all,

I'm new to Power BI. I have got a few tables in Power BI and try to link them correctly and do some basic routine calculations. 

Now I have two tables, one lists Incidents data and another lists Number of staff data. Both of them have fields like Region, Cost Centre, Financial Year etc. I also have a third table that contains cost centre hierarchy if needed, to link these two tables by cost centre codes. 

In Incidents data, I can count how many incidents by Region by Financial Year. In staff data, I can count how many staff by Region by Financial Year.

Now I need to calculate the incident rate = No. of Incidents / Number of Staff. 

I need to also be able to break down the incident rate by Region, Cost Centre etc AND by Financial Year. 

I find the rates I get are not correct. My DAX is "

Incident Rate = COUNTA(Incidents[Incident No.]) / SUM(Staffs[Total]) * DISTINCTCOUNT(Staff[Report Date])

"

How should I do it to make it work?

Thanks in advance for any help!