Using Monthly Tabular Employee Data to Calculate Turnover|Dashboards & Charts|Excel Forum|My Online Training Hub

I'm building several HR dashboards that will have data added monthly. I am currently struggling with the best way to tackle calculating turnover. I have the monthly source data laid out in a table (see attachment for example) and am trying to figure out how I can get turnover to calculate into a pivot table, or even the best way to calculate it in the source data/table that I can then pull into a pivot table. I'd like the calculations to update based on the slicer selections. The formula we use for turnover is: The employee count at the beginning of the period + the count at the end of the period /2 to get the average, and then divide the total number of terms in the period by that average. I wanted to get some opinions on the best way to tackle this/how to setup that formula. If you had 12 months of employee data laid out like I have it in my example, and needed to pull turnover %, what approach would you take?

*6/27 Update* 

I had some time today to throw things at the wall and see what stuck. I learned about measures today, and started playing around with those. But, I always get issues when using power pivot and will get "An issue with the Data Model is preventing Microsoft Excel from opening this workbook. Try restarting Microsoft Excel" - which popped up on me again today. (I just updated to 365 too, hoping I would have less issues.) But, before it blew up on me, I had figured a couple measures (thanks to google) that I was able to combine to get the average count divided by the terms and it would update when I filtered the periods. Now, I need to figure how to get it to calculate based on only the beginning and end date of selected date range. I may just put my start and end dates in an external cell, add a cell reference to my calculation, and not give the option to slice/change periods. But, I'm still open to suggestions. I feel like there's a simpler way to tackle this, but I've only been truly using excel for 3 months now and I'm learning there's like 10 ways to do everything, so any help would be greatly appreciated!