on the 8th day of a flu it almost ran its course by itself. To really strike back you would have to start with vit C right when you felt its onset 8 days ago..
However, titrating vitamin C has many other advantageous effects therefore its really never too late. And since pure ascorbic acid is the most potent form for this purpose, I would add as little baking soda as to make megadoses of ascorbic acid reasonable palatable to you. For me and 8 gram ascorbic acid that's only about a 1/2 gram of baking soda. And even less soda, if I use some fruit-juice instead of pure water.
PS: Because titrating vit C at a onset allows no hesitation, I would recommend everyone to experiment with titrating vit C before a flu happens. Since only then one understands much better the symptoms of getting closer to bowel tolerance. And if questions arise no need to wait more hours till an answer appears on a board.