The great thing about Campden tablets (a convenient form of dosing in sulfur dioxide for home winemakers) is that they will inhibit the yeast and bacteria you do not want (which are sensitive to sulfur dioxide) while allowing the yeast you do want to continue to power through the fermentation.
Longtime readers of my column know that I almost always inoculate my wines with a commercial yeast strain (purchased from a reputable dealer) that is specifically chosen for my wine’s conditions. Since you’re doing a red wine in the Prosser area, I’ll figure you’ll be able to get your grapes plenty ripe . . . and that you’ll need a yeast strain that can tolerate a temperature up to about 95 °F (35 °C) and an alcohol up to 15%. This will provide the best chance at a complete fermentation.
A purchased yeast is grown and formulated to tolerate a little bit of sulfur dioxide and I almost always add around 35-50 ppm total sulfur dioxide at the crush pad in order to inhibit ambient bacteria and to suppress “wild” yeast that could spoil an entire batch. By dosing your grapes with a little sulfur dioxide, you’re making sure your yeast will be able to power through your fermentation safely while “bad bugs” and their “bad behavior” will be repressed.
One crushed Campden tablet in one gallon (3.8 L) of wine produces about 65 ppm (or mg/L) of total sulfur dioxide. Since I’m not sure which recipe you’re referring to in your question I can’t quite tell you if one tablet is too much or not. Just be sure you don’t use more sulfur dioxide or Campden tablets than a recipe calls for. Measure twice, cut once, and ferment on!