That percentage is a percentage of the total dividends. So, you multiply by the box 1a (ordinary dividends) of the 1099-Div.
The box 1b amount is already included in the box 1a amount, so you don't add them together. Nor, do you subtract 1b from 1a.
After entering the 1099-DIV, the next screen will ask "tell us if any of these uncommon situations apply to you". One of the choices will be "a portion of these dividends is US government interest". Check that then continue and the following screen will have a box to enter the amount (not the percentage, you'll have to do the calculation)
