Matrix Multiplication

[ChiccoLisoFolmichino]

Hello Guys,
I am setting up an excel workbook that will allow me to have a simple forecast of future customer conversion.
I am going to base it on a function of the historical behaviour of customer.

The sample I have attached is working on a yearly basis. Since I may need to go in further details on a monthly basis, I am looking for the proper formula in order that my matrix will automatically work.
I had help more than one year ago from FlameRetired in this post while doing a customer analysis.
Unfortunately, due to a lack of comprehension of array formulas, I wasn't able to adapt it to my new matrix.

In the file you will find in red my comments relatively to where I am stuck at.
Thanks a lot :)