?Hi, ps56K.
Looks to me like you are mixing apples and oranges and winding up with something that is neither applesauce nor orange juice. :>{
In my former career as a tax accountant and for my own investments, I've often used Quicken to compute gain or loss for tax purposes. In most cases, the corporate acquisition "wizard" in Quicken does a good job, once we realize that when it asks for the "cost" of the new shares it really means the FMV (Fair Market Value immediately after the acquisition), since the new "cost" basis is what we are trying to have the wizard calculate.
But, as you've noted, this new cost is not useful in computing Return on Investment or other financial statistics going forward based on events and values after the acquisition. For that, you need to use the acquisition market value as the basis (not the cost basis) for your calculations.
Early in my 20 years of using Quicken, I gave up on understanding its interpretation of ROI and other financial ratios, so I almost never have it perform such calculations and I can't tell you how to do them with Quicken. But I THINK I would simply change the cost basis and starting date to the Berkshire shares' value on the date of acquisition, then make my financial return calculations, and then revert to the tax value and date. But I'm sure that others here can give you better guidance on how to accomplish your investment calculations without messing up your tax records.
It does seem to me that both ROI calculations can be useful for different purposes, just as year-to-date results are useful, in addition to results for the full holding period. But make sure you know which is which and don't mix them and end up with orangesauce. :>(
RC