Mere millionaires in California are now eyeing the exits over fears that a billionaire tax could be tweaked to target smaller fortunes, a tax advisor to the mega-rich revealed. The proposed 5% wealth tax on billionaires was just cleared for the November ballot this week — but California’s millionaires were already spooked about the ramifications of the controversial proposed levy , said David Lesperance, a tax advisor to high-net-worth families. Now the tax guru told The Post that it isn’t just