Help! Dynamic Top N for Cross Tab group sort Epert

I have using Cross Top in my report. I want pass the Top N from my VB.Net code to the Top N. I have set the formula _TopN in the report and assign it in the Cross Tab Group sort expert. But the number pass from my code to the formula not work. I pass in 1 to the formula _TopN but it keep show me 10 record which default set in the report.

Any one have any idea or work around.

Below is the code how I pass the number to the formula

rpt.DataDefinition.FormulaFields("_TopN").Text = CInt(sTopN)

Please Help!

Thanks.