well it does actually depend on your tastebuds and your sweetness.  Generally I usually use half (because things taste so sweet to me), however to being with I just replaced as it, so 1 cup I used in place 1 cup.  If your using dextrose then you may need to add some more liquid (dextrose tends to soak up a lot of liquid) and you may need to adjust the cooking or the temp as I find dextrose cooks so mcuh quicker.