Overview of how the voting-eligible population (VEP) is constructed.
The voting-eligible population is constructed by adjusting the voting-age population for non-citizens and ineligible felons, depending on state law. National estimates are further adjusted for overseas eligible voters, but no state level adjustments are made since their is no reliable method of apportioning overseas voters to states (except 2008, as described in this FAQ).
There are adjustments that I am unable to made due to lack of available data, such as the number of permanently disfranchised felons, depending on state law, the number of persons who have been judged mentally incompetent by state law, and the number of persons who have moved within a state after the close of voter registration, depending on state law.
Note, if you have moved between states after a registration deadline federal law guarantees you the right to vote for president in your former state (see 42 USC § 1973aa-1(e)), a little-known policy known as "Presidential Ballots". Many states also have provisions that permit voting if a person has moved within the state or locality and have not updated your registration address. Please check with your local election administrators for more information regarding your state's laws.