When putting on a Tallis in Shul, Kaddish is sometimes said, and I didn’t quite wrap it fully over my head. When am I able to answer Amen Yehey Shmei Rabba, and not have said, a Bracha L’Vatala?
As soon as the upper part of your body and head are surrounded with the tallis, you can answer Amen etc, even if it is before you drape all four corners over your left shoulder. The reason for this is since according to the Shulchan Aruch (O:CH 8-1), you have fulfilled the mitzva when you are surrounded by the tallis, and there is no need to gather all four corners and drape them over the left shoulder, (called Atifas Yismaelim). Although it is widely accepted to do Atifas Yishmaeilim, however it is definitely enough to consider it that you were mekayem the mitzva already and it isn’t a hefsek between the bracha and the mitzva.
Also see Gr”a O:CH 8-4, Maaseh Rav 15, Chayei Adam 13-3, Halichos Shlomo 3- ftnt. 33.