Dividends from mutual funds in showing foreign taxes paid, seem to be a combination of domestic and foreign dividends - but they don't break it down. How to calculate %?

[Patricia P]

And why is this % calculation (domestic vs. foreign) an important part of filling the form?

[SusanY1]

Many mutual fund companies now provide supplemental dividend information that will give you the breakdown of foreign and domestic dividends paid.  If it is not on your tax papers, it may be in the mutual fund's performance documents or other materials published by the fund. 

It matters because  the total income plus the foreign only income are required to properly compute the credit.