National opinion surveys show Ross Perot drawing up to a quarter of the vote as an independent Presidential candidate. But the data-processing billionaire may have the computer-nerd vote largely to himself. An on-line poll conducted by Computer-Based Communications, of Lexington, Mass., shows Perot getting 41% of the vote, compared with 30% for President Bush and 29% for Bill Clinton. It may be a while, however, before this new method of polling catches on. Only 140 users in the Northeastern U.S. have so far called CBC's Argus bulletin board to voice their opinion.