According to Brewers Friend 300g would add 0.72% in a 5 gallon batch. I highly recommend using that site for free and easy to use calculators https://www.brewersfriend.com/stats/
Use the complete recipe builder, add in your grains or liquid malt by weight and batch size to get a target SG. You can add other fermentable's such as sugar or honey. Also if you know what yeast your using then it will give you an expected FG.