So I know we've discussed ET a ton here now, especially the magic, but there is a very interesting line I just noticed;

In Khaine, It states in the first paragraph of End Times Spells that the spells "cannot be dispelled if successfully cast." That seems to very strongly imply that successfully cast is simply meeting the casting value. Of course it could very well be more unintentional, crappy writing, so make of it what you will