I just use a dash or so of salt, but there are many websites devoted to explaining how to calculate sweat-rate and sodium needs. (I tried googling 'calculate sweat rate cycling' , but having trouble getting websites to load.
Edit: here is one. Don't think you have time to do it before tomorrow, though. There are some others that just mentioned body weight and average exertion. The point is, these organic smoothies and such typically have too little sodium, so some needs to be added, maybe a dash or two of salt, and if you are doing a century, you're not going to be able to refill unless you are able to carry a lot on your bike.
I carry two 25 ounce Camelbak bottles on the bike. Much easier. One plain water, the other with electrolytes or caloric beverage.