DataTable Expression

I am trying to set expression for the column Lower and Upper
 Formula
cpk = Min[ ( P50 - LSL / 3(P50 - P5)/1.645), USL - P50/ 3(P95 - P50)/1.645) ] 

p50 = Median

Lower.Expression = "(Convert(Median,'System.Double')-Convert(LSL,'System.Double'))/(((3.0)*(Convert(Median,'System.Double')-Convert(point5,'System.Double'))/(1.645))";  
//Error The expression is missing the closing parenthesis.

Upper.Expression = "(Convert(USL,'System.Double')-Convert(Median,'System.Double'))/(((3.0)*(Convert(point95,'System.Double')-Convert(Median,'System.Double'))/(1.645))";

CpK.Expression = "IIF(Lower < Upper, Lower, Upper)";