How is the average buy price of your open position calculated when you partially sell shares, whether the sale is at a profit or at a loss?

The average price is determined using FIFO (First-In, First-Out) logic.  

This concept applies when the same stock is purchased in multiple tranches. For example, if you buy 100 shares of ABC Ltd at ₹1000 and later purchase another 100 shares at ₹2000, then sell 100 shares on the same day, the sale will be adjusted on a FIFO basis as per first purchase. The remaining holdings will reflect an average price of ₹2000. 

The logic is simple - Profits or losses made on a particular trading day are settled to your ledger on the same day and you start the next trading day with remaining order value and quantity.