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Large white factor alone isn't enough to cause a dead foal; you need to have a lethal Kit cross present first. Large white factor just increases the possibility of a dead foal if you're combining two potentially lethal Kit genes. Here is the announcement post about Kit mutations and lethality. If you look at the google doc that's linked to right underneath the colored box chart, you can see detailed information about potentially lethal Kit crosses and which levels of white factor will give you a dead foal. The white factor levels indicate the white factor of the potential foal, not the White factor of the parents.

http://hj2.huntandjump.com/forum/discussion/13757/new-kit-lethality-and-kit-mutation-load-explained#Item_12

Non Kit lethal crosses like hom frame, hom splash 2, hom splash 3, and splash 2 splash 3 are not affected by white factor; they're just always hom lethal.