I just substitute the same amount, so a cup. But if you want things to have a similar level of sweetness you'll need more, and then you might need to change other ingredients... too hard really. I made baklava yesterday with dextrose and rice malt syrup instead of sugar and honey and it was lovely, yes less sweet, but it's usually ridiculously sweet. I do find that sometimes things need to be cooked at a cooler temp or a shorter time with dextrose.
