I am trying to track holdings in several retirement accounts where the
average cost method is the appropriate one. I have read the various
documents and discussions on the topic I managed to find in the Beancount
universe. I decided that "NONE" booking with manually computed average
basis values will do fine for me until an AVERAGE method is implemented.
(Thanks for the workaround, and for Beancount, of course.) I also
discovered and started using the check_average_cost plugin. In the process,
I encountered a shortcoming in check_average_cost -- or in my understanding
of average-cost basis computation.
My accounts have two types of reductions: 1. The common
sale/redemption/transfer out or 2. administrative fee paid in fund shares.
It seems to me that the two reductions have very different effects on the
cost basis of remaining shares even though the Beancount posting for the
account is identical. I set up a simple example where I buy 20 shares of
MFUND1 at $10 and sell 4 at $12. I repeat the same operations with MFUND2
at the same prices except that the second operation, the reduction, is a
fee payment in shares.
In the first case, the average cost basis for the 16 shares remaining after
the sale is $10. Cost basis is unaffected by the units or the price of the
sale -- as one would expect from average booking.
In the second case, average the cost basis of the 16 shares is $15.5
(($20*10+$12*4)/16) because the proceeds from the "sale" are a fee paid to
the administrator and thus increase the total cost of the remaining shares.
Such reductions as a fee to the administrator always increase the average
cost of the remaining shares.
At least, this is my understanding of how average-cost basis works.
However, the check_average_cost plugin does not agree. To illustrate, I
added third operation to both sequences above, a sale of 10 shares at $14.
Here is the entire file:
plugin "beancount.plugins.check_average_cost"
option "operating_currency" "USD"
1900-01-01 commodity MFUND1
1900-01-01 commodity MFUND2
2019-01-01 open Assets:Bank:Checking USD
1980-05-12 open Equity:Opening-Balances
2019-01-01 open Assets:Investments:MFUND1 MFUND1 "NONE"
2019-01-01 open Assets:Investments:MFUND2 MFUND2 "NONE"
2019-01-01 open Expenses:Fees USD
2019-01-01 open Income:Gains USD
2019-01-01 * "Opening Balance for checking account"
Assets:Bank:Checking 10000.00 USD
Equity:Opening-Balances -10000.00 USD
2019-01-02 * "Buy 20 shares of MFUND1 at 10 USD"
Assets:Bank:Checking -200 USD
Assets:Investments:MFUND1 20 MFUND1 {} @ 10 USD
2019-01-03 * "Sell 4 shares of MFUND1 at 12 USD"
Assets:Bank:Checking 48.00 USD
Assets:Investments:MFUND1 -4 MFUND1 {10 USD} @ 12.00 USD
Income:Gains
2019-01-04 * "Sell 10 shares of MFUND1 at 14 USD"
Assets:Bank:Checking 140.00 USD
Assets:Investments:MFUND1 -10 MFUND1 {10 USD} @ 14.00 USD
Income:Gains
2019-01-02 * "Buy 20 shares of MFUND2 at 10 USD"
Assets:Bank:Checking -200 USD
Assets:Investments:MFUND2 20 MFUND2 {} @ 10 USD
2019-01-03 * "Pay account fee of 48 USD in MFUND2, 4 at 12 USD"
Expenses:Fees 48.00 USD
Assets:Investments:MFUND2 -4 MFUND2 {10 USD} @ 12.00 USD
Income:Gains
2019-01-04 * "Sell 10 shares of MFUND2 at 14 USD"
Assets:Bank:Checking 140.00 USD
Assets:Investments:MFUND2 -10 MFUND2 {15.5 USD} @ 14.00 USD
Income:Gains
This fails bean-check with
...:41:Cost basis on reducing posting is too far from the average cost
(15.5 vs. 10)...
2019-01-04 * "Sell 10 shares of MFUND2 at 14 USD"
...
Changing the 15.5 to 10 satisfies the plugin.
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