Have subtractions to Investments:Dividend Income be recorded as subtractions instead of uncategorized. These are generally subtractions to foreign stock dividends and are deducted from the dividend category
When I had foreign stock dividends, I recorded the total dividend, and then a MiscExp for any taxes withheld rather than trying a "negative dividend".
By using a MiscExp, I could accurately select the category for the taxes, rather than merely reducing the dividend … which wouldn't be accurate accounting in any event.