(NapaValleyRegister)- Now more than two decades old, the rule that Napa wine be made with at least 75-percent Napa grapes is a signature of the wine industry. But two wineries’ recent proposals to expand production has called into question that rule’s future viability. What happens if wineries’ production exceeds the availability of Napa County grapes and crashes headlong into the 75-percent rule? Industry groups say it’s far too early to render any verdicts on that fear. They are in the early stages of analyzing the data and trends in grape and wine production.