I can place an order at 9.15 1s or 0s, using the API. The maximum change I can incur is +0.4%, kindly give your opinion on how I can do this? I use kite zerodha
Usually the best way to buy a stock at the open price is to participate in the opening auction. The issue is that you do not know in advance what the price will be (nor the exact match time) but you can place the order during the pre-open period.
If you want to buy just after the opening auction has occurred then the standard way is to listen to the market data feed and check for the open status for the feed to trigger your buy order.