The principal activity is considered to be in the country in which the worker maintains
a fixed base for more than 183 days in the taxable year. If the worker maintains a
fixed base in neither the United States nor France for more than 183 days, or maintains
a fixed base in both countries for more than 183 days, the worker's country of principal
activity will be considered the one in which he or she is physically present for the
greater number of days in the taxable year.