No one can know exactly how many voters will vote in each state on November 6, 2012. But if one uses the November 4, 2008 election data to estimate how many voters will vote in each state, one can calculate the percentage of voters who will see various presidential candidates on their ballots.
For Gary Johnson (assuming he is on in Pennsylvania, which is likely) the figure will be 95.08%. For Jill Stein, it is 83.10%. For Virgil Goode, it is 49.91%. Corresponding data for all presidential candidates will be in the October 1, 2012 printed Ballot Access News.