a question about chaptalization

I have a batch of freshly crushed touriga grapes, that I plan to turn into a port. It has an initial brix level of about 25, but I'm planning on adding enough sugar to get the alcohol level up to the limit of the yeast that I'm using (about 18%). When I compute the amount of sugar to add, should I base it on the expected volume of the finished wine, or should I base it on the volume of the current must (which obviously contains seeds, skins, etc.