It does work. Our well-known international tax consulting firm has done this for years. Shouldn't you know what amount of taxes were paid in a calendar year to a foreign government? Assume your only income is wages - you should know how much was deducted from your pay. You take that credit. Eventually, over time, as taxes are filed, values determined can be adjusted to see the actual balance. My tax return shows for the past 10 years the amount of foreign taxes I paid, claimed, and my balance. I have a positive balance because in the past I have claimed less than paid.