If you really want to try it, try it half and half.
The ratio of xylitol to sugar substitution is: 1:1 by volume
Supplement: xylitol is only a sweetener after all, and there's still a difference in taste from sugar; and xylitol is slightly minty and refreshing. Personally, I've never been a fan of sugar substitutes, just because of the taste. Which is the drink those, always feel with sugar substitute, feel powdery, not refreshing.
Foreign baking sugar substitutes, using a mixture of saccharin and sugar, as well as a mixture of sucralose and sugar; interested in trying.