Splits for tax-deduction tagging without totalling the entire check amount in Transaction Reports?

How can I use splits to effectively split a transaction between tax deductible items and non-tax deductible items without it totalling the entire check amount in the Transactions report?

Let's say I go to Wal-Mart and spend $300 with my Visa. Now lets say $200 of that is family living expense, and NON tax-deductible, and $100 was spent on office supplies, which ARE tax deductible. What I want to do is (either with tags, or categories, I don't really care which) be able to generate a report for my transactions that shows me all my tax deductible expenses with an accurate subtotal at the bottom by month, for all accounts. I have been just tagging everything that's deductible, even in splits (only tagging those items within the split that are deductible, leaving the ones that aren't blank, and not specifying a tag for the entire check). This works fine, except for the subtotal being incorrect. In the Transaction report, it only lists the items that I have tagged as "Tax Deductible", but in fact subtotals the split items in the list for the WHOLE check amount on that transaction. So with my above example: It would add in $300 to the tax deductible transaction report, instead of only the $100 (the split of the transaction which I've tagged as deductible. Is this intended behavior or a bug, or perhaps I'm just not understanding the purpose of splits? Any insight would be greatly appreciated.