Example: A US Citizen files a joint tax return in France with their nonresident spouse. In the US, the US Citizen files claims married filing separately status and files Form 1116.
On the French Income tax return, the US Citizen’s net salary is €100,000 and the non-resident spouse’s net salary is €50,000. They report foreign income taxes paid of €33,000.  It is necessary to allocate the French income taxes paid to arrive at the proper amount relative to the US Citizen’s income:  (US Citizens’ Income / Total Income) x French Income Tax = Income tax allocated to US Citizen.  (€100,000 / €150,000) x €33,000 = €22,000  The US Citizen would convert the Euros to US Dollars using the average exchange rate for the year. This amount can be claimed as foreign taxes used for credit.