Hi,

One of the options to achieve this:

1. You can have a new model with fields, employee_id, date (will represent month) and bakes made during that date
2. You can add this new model as a one2many to your employee record, and each month you add lines per month how many cakes made
3. In your Salary rule, you need to add/modify based on your needs, it will calculate how many cakes are made for that employee by checking your inputs from one2many field in Employee record.

More or less, you can achive with this try