Reporting Services - Giving a formula through Parameters

I'm using SSRS 2008, I'm building big reports and I use subreports, I need to give a formula by paramaters to a report. How can I do it ?

I though about doing reflection on the 开发者_如何学编程parameters in order to use it like a formula. Do you have any ideas if it'll work ?

Regards,

ps : sorry for my bad english.