Let's say that you have $1 million in your IRA that is is $600k in stocks and $400k in bonds and that you also have $250k in CDs outside your IRA and that the IRA earns 5% and the CDs earn 1%.
The way I do it (and I think many others) would be that the AA of that person's total portfolio is 48/52 ($600/$1,250 and $650/$1,250) and the return is 4.2% ($1m@5% + $250k@1%... or $52.5k/$1,250k).
If that person thinks that their portfolio earns 5% because they look at their brokerage statement and it says 5% then IMO they are fooling themselves because they are conveniently ignoring that they have a substantial investment in lower performing assets.
So when it comes time to rebalance I would rebalance to a 48/52 AA across both accounts.
I don't include our homes or cars or anything like that in the portfolio... like you say... just investable assets.... but would include any financial assets that we have the we plan to or could be used in retirement so taxable accounts, tax-deferred accounts (tIRAs, 401ks 403bs, etc) and tax-free accounts (Roth IRAs, HSA's, etc.) but excludes our local bank accounts that we use to pay our bills that typically have de minimus balances so have no significant impact, homes, cars, boats, etc. In some situations I can see where college funds might be carved out and viewed separately, but I chose not to.