Incremental Increases based on criteria

Hi

I am working on a model looking at the launch of a new product. On the attached file I am using the sales inputs tab to say how many distribution points (DPs) we will start with, which month a product will be launched into a particular outlet type and how those DPs will increase during 2020 - 2023. Lastly the frequency column will say how often the increase will take place. So for example the product will be launched into outlet type 1 in Jan 2020 starting in 20 DPs. It will increase by 10 DPs every month in 2020 then increase by 20 DPs every month in 2021.

The formula I have come up with, following much google time, works a dream but goes wrong when the year ticks over and I cannot work out why. The increase of 10 is fine in 2020 but in Jan 2021 it jumps by 130.

Anyone have a suggestion?

Many thanks

Chris