You must understand the requirements for your product, and analyze how these requirements broadly map to the Engine's key decision factor/key event concepts before starting implementation.
Unlike the requirements for eligibility/entitlement, typically requirements for key decision factors are not enshrined in legislation, but instead are based off informed predictions of the kinds of information that may be useful to a case worker, when the case worker is trying to understand a complex case (and perhaps answer questions about the case from customers.)
As such, producing requirements for your key decision factors is perhaps more art than science. For a new product, you might consider revisiting your key decision factor requirements once a product has been live for some time, in light with the kinds of information that case workers are attempting to understand when they view determinations. If you layer your requirements and implementation according to the recommendations, you should be able to implement and deploy changes to your key decision factor rules without affecting any underlying eligibility/entitlement calculations.
The following steps should aid your analysis.