Incorrect Fractional Calculations

Hi all, could you please help me understand the calculus of the following fractional transaction I made today, I’ll give you all the purchases I made so you have the whole picture.

Today I purchased TSLA at 0.1203148 @ 872.71 which is $105. I already had additional factional shares before today’s purchase. In summary, I bought

TSLA 0.0218188 @ 482.61 on 8/31/2020 ($10.53)
TSLA 0.0056442 @ 354.34 on 9/8/2020 ($2.00)
TSLA 0.0226911 @ 440.7 on 9/15/2020 (10.00)

Making a total of 0.0501541 @ 449.21 ($22.53), if we add today’s purchase that will be a grand total of:

TSLA 0.1704689 @ 748.11 ($127.53)

Later today I found a new stock I wanted to try and I sold 0.1203 of TSLA, I couldn’t sell the exact amount I purchased because the app doesn’t allow more than 4 decimals. The result operation was:

TSLA 0.1203 @ 881.70 ($106.07) which in theory I won $1.07, and as per my math that means today’s purchase was only 0.0000148 @ 872.71 ($0.01) having a total of:

0.0501541 @ 449.21 + 0.0000148 @ 872.71 = 0.0501689 @ 449.3385 ($22.54)

However, this is not the case as you can see in the screenshots below the result of the transaction was a profit of $16.07


and now my average cost is:

Do you think this is the expected behavior or should I open a support ticket? I’m not losing money but my profit reduces $15 which seems they were invested in TSLA.

Please let me know if this makes sense or if you want me to clarify something else.

I think this is where your maths went wrong, you sold from the total not from the last purchase, so you can’t deduct he sale amount from today’s buy.

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I see, thank you for the clarification, I didn’t know how the fractional buy/sell worked with multiple transactions, now it makes perfect sense.