Aggie, I'm not sure I understand your question but I understand how the foreign tax credit works.
So maybe this helps...
If you have a little bit of credit ($300 or less if single and $600 or less if married file joint), you don't have fill out an 1116 form and so simply get to deduct your foreign taxes from your US taxes. This also means if you're under those thresholds, your tax rate doesn't matter.
If you have to file the 1116 form because you're over these thresholds, you can use the foreign taxes to wipe out the US income taxes you would have paid on the foreign income... but not on other income. In this situation, tax rates matter...