Wouldn't it be more direct just to calculate the after tax proceeds assuming Apple was sold completely?

Sure, though I would add a proviso:
that it was sold at a "fair" valuation level, neither unusually high nor low. We're talking value, not price.

Another quibble might be that if you consider it an effectively perpetual position, the tax liability is pretty irrelevant--you really want only the present value of all future distributions.
But we can leave that to the geeks.

Still, either of those would only get you the gross value of the position, not the fraction of the value of a Berkshire share.
Berkshire doesn't do a lot of it, but they do borrow money to make investments: there are very close to $2 in assets for each $1 in equity.
On average, I think of the percentage value of one of their investments to a shareholder as the value of the shares minus the proportional value of the liabilities.
In round numbers, any random 10% of their assets (counting its 10% share of liabilities) corresponds to 5% of the firm's value.
That sounds weird, but if you follow the logic you see why:
If you add up the value of all the individual assets, you get way more than the total value of the firm if you don't count the liabilities each time too.
If it's necessary to consider the liabilities for the sum of all positions, then it's necessary to consider it for any single big one when assessing its percentage value.

Another way to look at the proportional value of the Apple position for Berkshire holders, for the pessimists:
What would be the percentage hit to Berkshire if Apple were a permanent zero tomorrow?
That would be a much bigger number. When it goes to zero, the proportional liabilities don't go away.

Personally, I do it this way--
What's the Apple position actually worth? I use a multiple of cyclically adjusted owner earnings.
What's the value of a Berkshire share? I use various methods for various subsets of the firm, then add 'em up.
Divide A by B to find out what fraction of the value of a share is attributable to the Apple holding.
I generally get a number around 1/6. You might also get around 1/5, depending on assumptions.

Jim