Here's a convenient list of the previous election years. https://en.wikipedia.org/wiki/Voter_turnout_in_the_United_States_presidential_elections
I'm thinking it would be best to have it centered at 55%, and go from 45% to 65% at 2.5% increments, with options for <45% and >65% for a total of 12 different options.
Voter turnout is an important parameter in a democracy. In 2016, we could get higher turnout, because more people follow politics today, or lower turnout, because both candidates have record unfavorability numbers.