IMHO there a number of issues embedded in the OP's question, most of which have been touched upon by the previous posts:
1) The whole point of investing in CCFs is to break out of the "2 dimensional" stock-bond paradigm by adding a new asset class to create a "3 dimensional" stock-reals-bond paradigm. Thus, each asset class should get its own allocation. (And although many people prefer to put the real allocation AFTER the bond allocation -- e.g. 60/30/10 -- I prefer to put it in the middle because it characteristics are closer to stocks -- e.g. 60/10/30.)
2) On the one hand the CCF allocation should "come out of" the stock allocation because you are substituting one volatile asset for another in order to reduce overall volatility. Thus a 70/30 portfolio might become a 60/10/30 portfolio. On the other hand, once you acknowledge a third asset class ("real assets") then really (no pun intended
) you should stop thinking of "robbing Peter to pay Paul" but actually develop a 3-way allocation that makes sense in its own right. This also requires that you define for yourself what the "real asset" class comprises. Some folks for instance consider REITs to be part of the real asset class; some consider them a stock sector. (IMHO, they exhibit both characteristics but over the medium to long term are a distinct asset class.)
3) In the case of PCRIX in particular, because the collateral is held in TIPS as part of PIMCO's "Double Real", the performance of PCRIX is the sum of the performance of DJ-UBS Commodity Total Return Index *plus* the performance of the TIPS portfolio. See: http://raddr-pages.com/research/CommodityFutures.htm
Thus some folks have been tempted to count their PCRIX allocation TWICE: once as CCF and once as TIPS! In other words, a case can be made that PCRIX should be counted as part of the bond allocation (thus adding to it rather than subtracting from it!)
IMHO, there is no "perfect" answer to the OP's question -- and no need for one. But there is need for both an approximate answer as well as an informed understanding of the shortcomings in the approximate answer.
In my own case, I divide my portfolio into 3 broad asset classes and include PCRIX among the "real assets", along with REITs and a few other things. My overall portfolio is roughly 50/25/25, which some might consider a little aggressive for an early 50's investor. To my mind, however, the third asset class really does provide diversification that allows me to have a somewhat lower bond allocation than I otherwise might have (especially since I have access to TIAA's Real Estate Account). Nonetheless, I am mindful of the fact that some of my "reals" have "stock-like" attributes and that some others of my "reals" have "bond-like" attributes. Thus, one way of viewing my portfolio (using the "cascading asset allocation" format -- http://www.bogleheads.org/forum/viewtopic.php?f=10&t=106505
[Large version: http://i46.tinypic.com/5c0qqu.jpg
By using color this way I am trying to remind myself that my stock allocation is really "47%-plus"
(and for some purposes might be considered as much as 58% -- or even 75%) and that my bond allocation is really "25%-plus"
(and for some purposes might be considered 35% [since TIAA REA has very low volatility] -- or even, at a stretch, 42%). The precise numbers don't matter so much to me as being able to remember/visualize the dynamics of the portfolio under various conditions -- e.g. stock market crash, war, inflation, deflation, flight to safety.
I realize that I am probably at the extreme end of the S&D spectrum and that not everyone would go along with this approach. But it makes sense to me and seems to have worked well over the past decade -- i.e. since I became a Boglehead.