You can make the above logic as a method and just pass the input value to the method.
arr1[i,9] = "GROUP_FIELD_NAME:CheckAmount";
arr1[i,10] = "GROUP_FIELD_VALUE:" + GetFormattedValue(reader.ReadElementContentAsString());
private string GetFormattedValue(string strVal)
{
return strVal.Substring(0, strVal.Length - 2) + "." + strVal.Substring(strVal.Length - 2);
}