Dear,
As your business rule is intended to be used directly in you business process, it's recommended for performance to not consume it as a web service reference. Plus it's much easier to integrate with.
You can create the ruleset directly in the same business process development component, and call it directly in the mapping expression of your process. the beauty here is you can also use the same schema types of your process context data object while defining your rule input and output, so it will be easy for you to map the input and output while calling the rule function. screenshot below, and under it the steps to implement this.
1- Create your ruleset in the same process development component, under Process Modeling --> Rule Sets (above screenshot).
2- In the ruleset definition, use the XML schema types of your process context while defining your ruleset input and output.
3- Add a mapping activity after your human activity "Enter Details" (you can add the expression directly in the activity output mapping).
4- In the mapping expression, locate the rule function under Rules and Functions --> your namespace (above screenshot).
5- The input of the rule function should be inputs from your dataobjects (simple or complex types are supported).
I hope this would resolve your problem.
Ahmed