In case of Delivery displaying average purchase price is ok, better if user has both options.
In case of Intraday, Avg. Buy/Sell & also BreakEvenPoint (BEP) should be displayed.
Example: consider following intraday trades of scrip - “ABCXYZ Co”
Please consider.
For Calculation the Loss or Profit, one has to use the method of IRR (internal rate of return) and not average pricing per share or CAGR. IRR accounts for all money invested including all the dividend earned (with the duration of holding) and money made when you sell the holding (whether in parts). The profit/loss gets added to your overall position on the date of sale. So it is actually the total value of the trade which is being adjusted and not based on the number of shares alone.
I find this the best method to identify if you are doing a good investment or not, and it is adjusted for annual returns for small investors like me who keep adding money to the portfolio. CAGR method gives no clue of this insights unless it is is one single purchase and one single sale. Sometimes when your portfolio value doesn’t move (because of the profit/loss of that day are equal or not majorly apart) IRR still decreases because of your holding period increase by one more day - which is the real gauge of your investment. And also gives you the picture of how fast your money made more money for you in the most efficient way. Oh God! I sound Like a Capitalist! 
But to answer the question first posted, IRR considers the remaining shares in your portfolio in FIFO order.
Note: Value Research Portfolio manager gives you the IRR (for individual holding and overall portfolio). Also, XIRR function in Excel can be used.
The lot which will be sold is defined by FIFO!
Re calculation is done only if he adds to current holdings.
In this case u have not shown any additional so avrage price wont change
Average price displayed would definitely be Rs. 200 as shares bought on previous occasion no more exist and hence what is left are 100 shares bought on 5th April which are bought at Rs. 200 
Using FIFO method of calculation will end me up in loss. As it will remove the previous information on buying
e.g Take the sequence
BUY 100 S @100 -> AVG Price 100
BUY 100 S @ 50 -> AVG Price 75
SELL 100 S @ 90 -> FIFO (AVG = 50) , Normal AVG ( 75)
FIFO - > During Sell order FIFO calculation will show it as profit of 15100. But in actual you are losing 10100. So when you are implementing FIFO based calculation then PNL should be recalculated for each trade. This is only good for tax purpose and not trading.
AVG -> In this case you can make well informed decision whether you are making profit including the transactions done in past. So I will always prefer AVG method for trading.
FIFO doesn’t apply here. Average weighted cost per share should apply here as shares are bought in different time frame. So in this example WAC/share is 100 X Rs. 100 = Rs. 10000,100 X Rs. 200 = Rs. 20000, so WAC/share is (10000+20000)/200 = 150, so if now I sell 100 shares @Rs.150, WAC/share will be Rs. 150.
Dear all, can anyone tell the answer to this? How zerodha calculates?
Fifo is used here. So the for the remaining 100 shares its displayed 200.