It is critical to know what the account margin requirement will be if the underlying position change value. For example, if sp500 drop 10%, will my account have a margin call? IB already has method to calucate real time margin requirement. IB can use the same system adding that the customer can enter underlying position value to calculate margin requirement in what-if situation. In this way, The customer can manage margin better. They will not trade less worrying about margin call, so that IB will earn more commissions. They will not use too much margin which will result in account liquidation, which makes current customer close/blow their accout and new customers hesitate to open an account with IB. IB please do this which benefit all of us and cost less.