Stock spinoff transaction in Gnucash

[Stuart McGraw]

I recently posted asking about how to represent a stock 
spinoff (where the owner of some stock gets some additional 
stock of a different company as the result of the original 
company "spinning off" some of their operations into the 
new company) in Gnucash.

Responses to that request and some googling turned up 
some suggestions but nothing very definitive.

After a lot of research and experimenting I offer the 
following for critique and if acceptable, a reference 
for others needing to do this.

I won't try to explain the methodology for determining 
the numbers to be used in any particular transaction,
but 
  http://www.costbasis.com/stockchanges/spinoffs.html
has a good explanation, and 
  http://www.costbasistools.com/spinoff/calculator.php
is a form into which you can plug in the basic parameters
of the spinoff and gives you the worked-out calculations. 

One should also note that the treatment above (and on which 
my method below is based) is specific to the requirements 
of the US tax code and may not be appropriate in other 
localities or in all situations even in the US.

To provide an example of a spinoff transaction in Gnucash
consider a hypothetical company ParentCo in which I own
50 shares at a cost basis of $18 per share...

ParentCo spins off SpinCo with 1 share of 
SpinCo stock for every 4 shares of ParentCo stock.
After the spinoff ParentCo is $25/share and SpinCo is
$20/share.  As I understand it, the calculations are:

 Spinoff ratio = 1 / 4 
 Adjusted Spinco share price = $20 * 1/4 = $5
 ParentCo + adjusted Spinco = $25 + $5 = $30
 ParentCo ratio = $25 / $30 = 5/6 

As mentioned, my basis price/share for ParentCo is $18/share.
The post-spinoff ParentCo basis = $18 * 5/6 = $15/share
The post-spinoff SpinCo basis $15 * ($20 / $25) = $12/share

Now suppose I have 50 shares of ParentCo.  Total pre-
spinoff basis is 50 shares * $18 = $900.
50 shares of ParentCo @ $15 adjusted basis = $750.
The difference, $900 - $750 = $150, is allocated to the
12.5 shares of SpinCo received so SpinCo basis is 
$150 / 12.5 = $12/share (which confirms what we already
noted.)

However, I receive only 12 shares of SpinCo and I get
the 1/2 fractional share (@ $20/share) as $10 as "cash-in-lieu".  
The basis of 1/2 share of SpinCo is 1/2 * $12 = $6 so 
$10 - $6 = $4 is capital gain.  The $6 is a non-gain
sale of the stock.

To capture this in Gnucash...

I assume there are asset stock accounts for ParentCo and 
SpinCo, a bank asset account for Savings (where I'll deposit 
the cash-in-lieu check), and a CapitalGains (long or short
as needed) asset account.

The transaction, in the ParentCo register, is (view with
a fixed width font):

  Account                          Shares Price   Buy     Sell
----------------------------------+------+-----+-------+--------+
Assets:Investments:ParentCo          -50    18            900.00* 
Assets:Investments:ParentCo           50    15   750.00*
Assets:Investments:SpinCo             12    12   144.00*
Assets:Savings                                    10.00
Assets:CapitalGain                                          4.00
[* - value calculated by Gnucash from # of shares and price]

I think this captures all the necessary details.

One problem is that this transaction will appear twice in the
PerantCo register.  However, since the same thing happens when
recording a stock sale by the Gnucash officially recommended 
method, it does not seem to rule out this method.