Normally At The Money amount is selected using the last traded price of underlying which can be Index / Spot or Future.
However, in some cases, that may not be the right value for an ATM. If you have ticked Implied Future, then bridge will calculate the ATM as explained below.
First, It will check the Day Open Price of the underlying and it will select the ATM strike as per day open price.
Now it will apply the formula below to get the implied / synthetic ATM.
Implied ATM = ROUND( STRIKE + CE LTP - PE LTP)
Eg: Suppose underlying was NIFTY 50 and day open was at 17520, so ATM as per day open will be 17500. Now the bridge will check the current LTP of 17500 CE (75.85/-) and 17500 PE (25.80/-) and then calculate the current as per above formula.
Implied ATM = 17500 + 75.85 - 25.80 = 17550.05 which will be rounded to 17550 and if Skip NIFTY 50 was ticked then it will become 17600/-