Ascorbic Acid is by far the best available form of Vitamin C as talked about by Linus Pauling and Robert Cathcart. Of course theres more natural forms like Rosehips but were talking common health food store stocks. Calcium Ascorbate is not very good at all really and the only other form of Vitamin C people should rely on is Sodium Ascorbate.
Sometimes we have to take some measures not quite in line with what we may think. I would certainly take as much Vitamin C as it took for Bowel Titeration and complete it till my cancer was gone just under this level. I know of someone who cured their stomach cancer with just Ascorbic Acid and Enemas.