IMHO, adding nutrients up front, and at the 1/3-sugar-depletion mark is a fine idea. I believe adding at the 2/3 sugar mark is a bad idea.
The ideal nutrient schedule really depends on the starting nutrients (specifically, the yeast-assimilable nitrogen) in the must. Without knowing that, you are just guessing. (However, what other choice do you have, short of making a large investment to measure YAN?) We assume that DB or SP will be low in nutrients, so we add the nutrients up front.
My understanding is that the yeast need the YAN while reproducing and growing. By the time 2/3 of the sugar is gone, and there is significant alcohol in the must, the yeast are no longer reproducing like crazy. They are hanging out, living out the rest of their days in an alcohol-crazed existence. Therefore, they won't/can't use additional YAN at that point.
So, sure. Either add it all up front, or add half up front and half at the 1/3 sugar mark. I believe you will be fine either way.