Hi Induja,
As your business rule is to calculate the approver, probably you want to use it 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- In the human activity potential owner expression, locate the rule function under Rules and Functions --> your namespace (above screenshot).
4- The input of the rule function should be inputs from your process context dataobjects (simple or complex types are supported).
5- The output of your rule will be a string for the approver principal id. so you can wrap the rule function call with getPrincipal call.
I hope this would resolve your problem.
Ahmed