You definitely want to water down that high-sugar juice before you pitch your yeast. High Brixes lead to high alcohols, which lead to yeast that just can’t complete a fermentation. Stuck fermentations are no fun as they almost always lead to wine spoilage. The resulting problems of microbial infection, high VA, and off-odors are truly tragic.
Luckily, such winemaking travesties can be largely prevented if we all just begin fermentation with strong yeast and reasonable Brixes. I like white fermentations to begin no higher than 24.5 °Brix, though I’ll always go lower, like to 23.0-23.5, especially if flavors are ripe and the acid needs to be higher like in a Pinot Grigio or a Sauvignon Blanc. Reds can usually go a little higher but don’t go too high.
How much alcohol is produced for every degree Brix? Would you believe that there isn’t an easy answer and that this is something winemakers struggle with every year, including me. One rule of thumb is to multiply every degree Brix by 0.56 but I often find that too low. 0.57 tends to be a more realistic conversion factor to me but actual conversion will depend on the yeast strain, fermentation temperature, yeast nutrition and even can vary from harvest to harvest. If you have a lot of fermentations going on over the course of a harvest, one strategy is to get an alcohol reading on the first batch that you have finished and calculate the actual conversional rate (keeping in mind any water you have added). Then use that number to help guide your water addition.
Home winemakers can add water at any time (commercial winemakers can only “hydrate” to facilitate fermentation and correct dehydration of grapes before fermentation). As with any addition, it’s best to adjust a wine’s macro-chemistry as early as you can. In the case of white wine, this is best done in the juice phase. With reds, it’s best to wait until after the wine has soaked up for at least 12 hours so you’ve absorbed more sugar (it’s often hiding in the skins and possibly “raisins” on the cluster) into the juice for an accurate test.
There are numerous online tools you can use, including some on winemakermag.com, to help you make juice and must adjustments. It’s very important to get the right Brix so you have a healthy and complete fermentation.