The issue you are having is because you are trying to reconcile an
income statement methodology with the balance sheet and there isn't a connection between the two. At some point/certain points, the company must "true-up" their allowance for doubtful accounts with what shows up in the G/L (on the balance sheet) with reality (or estimated reality) & it is NOT uncommon to have out-of-cycle JEs that will adjust expense and the allowance account if the estimated % of bad debts isn't accurate. This is generally referred to as a change in estimate (it's possible to be an error if facts available at the time the estimate was made weren't considered).
Therein lies the key to the entire process: the estimated % used in the credit sales method is the single most important number, as that controls everything, including the expense that ends up in the P&L. This estimate is often so important, companies will disclose such as a Critical Accounting Estimate (we do). Outside of the aforementioned out-of-cycle JEs, nothing should touch the P&L, just the amount calculated from this %.
Note that the, "aging method" for estimating uncollectible accounts has the same weaknesses discussed above, it just generates the numbers differently.
In case you are interested, let me give you a real-life example.
Most of the time, the single most important item when evaluating uncollectible AR is a detailed review of the AR aging for balances outstanding much longer than the terms agreed to at the time of sale. For instance, if you are supposed to be paid within 30 days and a specific invoice is outstanding 90+ days, I'd say that's something your AR manager should specifically be looking at and informing you of. When an auditor comes out and audits the reserve for AR GL account, it's VERY common that they will closely examine old items and evaluate if it's reasonable that they are reserved for or not. This is much more of a "balance sheet" approach.
This is exactly how I do it, except I have a specific person in AR that follows up on each and every AR balance that goes over 40 days and, on a monthly basis, everything over 40 days outstanding is specificially identified and considered for reserve by my AR manager.
I'm not out of the woods yet though, as there's one thing that isn't being considered yet. Do you know what it is? If you guessed that there are balances in AR that are "current", but some are likely to get "old" and need a potential reserve, you're correct. Here, we use a percent of sales methodology.
What does this all mean? Well, we literally use both a balance sheet and an income statement approach to resolve our AR reserve challenges.
You cannot simply debit bad debt expense because it is now 2009, and that account write-off is related to 2008
Sure you can, especially since you now know your previous estimate wasn't correct. As stated above, this is generally a change in estimate or, perhaps, an error. In theory, it's a cutoff issue, which is exactly what's driving such change in estimate/error.
Wouldn't it be smarter to use a combination of the direct write-off method (for accounts that are related to current year sales that are written off) and percentage of credit sales?
Yes, as stated above, but I wouldn't worry about whether or not the reserve is for current year sales or not. If you believe you are going to collect it, don't reserve it. When/if the facts change and you think you won't collect it, reserve it. I wouldn't worry about what fiscal year that lands in; it's simply a change in estimate.
And your last statement is true regarding the aging method; the textbook indicates that the percentage of credit sales method is primarily concerned with estimating bad debt expense, as it is an income statement approach; any balance in the allowance account is ignored when determining bad debt expense under this method.
This is correct. See discussion above for explanation