Thank you for including a packaged workbook!
Are you wanting something like the sheet "Turnover Calc" in the attached? The keys here are to use Level of Detail (LoD) calculations for [Average Headcount] and [Terminations]. In [Terminations], there is the addition of the condition that counts only if [Total Terms] = 'Termed'. Then, the [Turnover] is simple division. I used INCLUDE type LoDs so it adds to the existing Days and Region filters. The alternative would be to use FIXED LoDs and add those filters to the Context.
It should be noted that the total terminations (695) is more than the sum of involuntary (393) and voluntary (301) because there is one record where the term type is null.
I hope that helps. Questions?