The accounting at the PWC level isn't complicated. All units owned and added or deducted are lumped together - so for simplicity's sake - I'll provide an example.
Let's say one owns X number of units at an average cost of $12 per unit - PWC will reflect that one owns a given percentage of the MLP - let's say .1%. One doubles up, at half the carrying cost, or $6 per unit. One now owns .15% of the MLP but the average cost is $9 per unit (averaging both lots).
At the end of the year the carrying cost is adjusted by PWC to reflect distributions, etc. The carrying cost for the .15% is now changed - no longer $6 per unit.
When one sells 1/3 of one's ownership each unit sold is treated as a sale of the percentage owned and the number you'll see on your next K-1 will reflect the disposal of 1/3 of your units and the appropriate gains, losses and adjustments against one's entire holding will be reflected on the K-1.
So ARB, to answer your question, you can't sell individual units to "take a capital gain" or loss. Your gains and losses are measured against your entire average cost of the holding.
The accounting and IRS theory behind this is that you can not buy or sell individual lots for tax purposes - these are not individual corporate stock shares. You buy to add to your aggregate ownership of the partnership and you sell a percentage of that melded holding to reduce your your percentage of ownership in the partnership.
I know that this sounds complicated but to any person with an accounting background it makes sense.