How to add customer attrition assumptions to a volume forecast?

Hi,

This is my first post so here goes!

I am trying to build in some attrition assumptions into a customer volumes data set and I am getting nowhere fast so would really appreciate some help.

- I have a set of customer volumes from Jan 2014 to Dec 2023 segemted into heavy, medium and low users
- I have some high level assumptions around annual attrition rates
- I have some high level assumptions around the cost of retention for each customer segment

What I am struggling with is combining everything to produce the attrition forecast. Ideally I would see:-

- Monthly and Annual attrition numbers expressed in customers and cost

I can do a basic high level annual calculation (attached) but really need a detailed monthly model, any help would be greatly appreciated.

Thanks