Dax Formula Assistance Sample Provided

Hello, 

So I'm attempting to calculate a rolling 12 months product ID attrition on a dataset. With the calculation as follows: 

Total active product IDs start of month  / Total active product IDs End of Month = Avg. Product Count 

Attrition = Total Product Discontinued end of month / Avg. Product Count  

Please see this excel file for the sample dataset. 

I need to be able to calculate product attrition by the product type within a rolling 12 months. I've been reading through the forums and some time intelligence guides out there. Several have been helpful in terms of guiding, but I'm struggling to get past issues and having a working DAX forumla to work with the desired outtcome. 

I have a date table in my Power BI model. 

I would be so greatful if somoene could assist me with this. Thank You so much in Advance.