The exact volume depends on, among a few things, how coarse the grains in your dextrose are, so it's hard to give an exact weight/volume conversion factor.
However, you are fortunate in that you know that you have 1 kg of dextrose in your bag. You can measure your own conversion factor that will be more accurate than a stock value pretty easily. What I would do is get two big bowls (or pots or whatever; make sure they're very dry). Dump the bag into one of them and use a whisk to stir it up. The idea here is to make sure it's not clumped together at all. Now get a measuring cup and measure from one bowl into the other to find out how many cups of sugar you have. For best results here, I would use a spoon to scoop the sugar and pour it loosely into the measuring cup. The idea is you want to avoid compacting it down because that'll be hard to replicate.
So now you know you've got 1 kg and it fills X cups (or mL or hogsheads or whatever your favorite volume measure is). Divide 1 kg by the volume and you'll have the density, which you can use to compute how many cups you need for 300 g.
That's overkill for a recipe, though. Just pour a bit less than half. If you're bottling, it is probably worth being more careful.