The total amount of the credit taken under section 901(a) shall not exceed the same proportion of the tax against which such credit is taken which the taxpayer’s taxable income from sources without the United States (but not in excess of the taxpayer’s entire taxable income) bears to his entire taxable income for the same taxable year. "
This is what @jtax was referring using a simple example of $25 foreign Tax . This implies that your allowable credit for the year will be based a ratio of tax on foreign income to that on world income -- it is taking an average tax rate to work this out. The point of this is that US can ONLY allow credit of the tax it charges / levies on the doubly taxed foreign income ---- it recognizes your current year taxes paid to a foreign administration and also the amounts that have been carried forward but allows only approx. what it would have levied on the foreign income -- thus it meets its obligation to ameliorate double taxation effects.
So for the example in he above post you would have current year allowable credit of $15 and $10 would be carried over . Note that if there are other treaty conditions, then there may also be adjustments to the foreign source income to allow for rate adjustments per treaty.
Is there more one of us can do for you ?