How do I use the targetType parameter in an IValueConverter?

This should work without too much overhead:

public object Convert(
    object value, Type targetType, object parameter, CultureInfo culture)
{
    double result = ... // your math

    return System.Convert.ChangeType(result, targetType);
}

you can do this

var typeCode  = Type.GetTypeCode(targetType); // Pass in your target type

if(typeCode  == TypeCode.Int32)
{
  // it is int type
}