I have created a Quicken file which shows that Quicken does not handle average cost sells correctly. It has 3 buys before a sell. The 3 buys are:2448.529 shares for $35,000; 3797.678 shares for $54,000 and 1047.236 shares for $15,000. The total is 7293.443 shares for $104,000 with an average cost per share of $14.26.Now the sell of 589.868 shares at $14.26/shares should reduce the total cost basis by $8,411.52. substracting this value from $104,000 gives a net cost basis of $95,588.48. This is simple math. The total cost basis that Quicken shows on the portfolio page is $95,568.25. If you subtract this value from $104,000 you get the cost basis that Quicken reduced the cost basis by, i.e., $8,431.75. If you divide this value by the number of shares sold you get the average per share cost basis that Quicken used, i.e., $14.29.I ran these numbers thru a spreadsheet and Moneydance(MD). MD did it correctly. Quicken did not.I am using Quicken Deluxe 2019, version 5.92 on MacOS 10.14.1.
My initial comment of using $14.26 was wrong it should be $14.41.