The overseas eligible population is drawn from various sources: Department of State Consular Services estimates of civil population abroad, DoD military deployment records, Department of State foreign service employees, and the Federal Voting Assistance Program. Unfortunately, Consular Services no longer published estimates of the private citizen overseas population. The Census Bureau provided a thorough overview of the problems associated with estimating overseas VAP which concluded conducting a census of overseas citizens impossible at this time.
State Level Overseas Estimates
In 2008 (only), I provide a state level estimate of the overseas citizen voting age population. I obtained estimates of overseas citizens from the Federal Voting Assistance Program, the federal entity created by the Uniform and Civilian Overseas Absentee Voting Act. These estimates are reported in letters FVAP sent to the states. These letters report statistics from the Department of Defense of all overseas military personnel who claim a state as their residence. My understanding is that the Department of Defense estimates that three civilian dependents are stationed with every four deployed personnel. FVAP then apportioned the number of civilians reported by the Department of State's Consular Services among the states using a formula based on the number of House seats within a state or territory. Note that FVAP - similar to the numbers reported on this website for other post-1999 elections - use Consular Services reports that are now out of date.
I modify the FVAP numbers for my calculations. I use the number of deployed military personnel as reported by the Department of Defense. However, I deflate the civilian numbers by 25%, which is the proportion of minors among the United States resident citizen population. I estimated -- using statistics generated from overseas voters who have registered with the Overseas Vote Foundation -- that the proportion of overseas registrants claiming a state as their home is roughly equal to the proportion of a states' resident citizen population, i.e., more people register in states with large populations. Instead of allocating overseas civilians based on congressional representation, I use a more direct proportion of a state's citizen population.
I only provided these state level estimates in 2008 since FVAP had made them available, and may do so again upon further investigation.