With so many ways to implement "calculate method on a web site". Which one are you alluding to?
1. Method just hosted as a plain response that can be consumed using AJAX.
2. Method exposed as part of a web service.
Following options you have to use these:
1. use HttpWebRequest to make a call to this page with relevant parameters.
2.a if it is a web service hosted on your machine under CLR (aka .NET Compliant service) it is a walk in a park. Right click the project in "Solution Explorer", click "Add Service Reference" and follow the steps as guided there.
2.b if it is a service hosted somewhere else or on some other platform/framework, e.g. Java, you can use wsdl binary to create a proxy class for it, which can used to invoke its methods.