Mixing Tylenol with formula

Ok so I've always just measured the Tylenol and then put it in her bottle of formula, and she drinks the whole bottle, so I assume she got all the Tylenol. But then I read you shouldn't do that except I don't see how it's any different from giving the Tylenol by itself and following up with a bottle. The only concern I could see is not knowing how much they had except I do know how much because she drank the whole bottle.....