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Coopers makes a great kit for refreshing ginger beer and adding one kilo of Raw Sugar to your batch is going to add about 2% to a 20L brew. The lactose won’t add much to your alcohol, but will increase the sweetness. Taste test once fermentation is complete to see if the brew needs extra sweetness. From my experience, you’re probably not going to need all, if any of the lactose. (+1 for what Bum said
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As a rule of thumb, I usually add between 2-3Kg of sugars to my Ginger beers to give them alcohol content between 5.0%-6.5% (20L batch). If you’re keen to get the alcohol content a bit higher, I’d suggest adding some more brown sugar or white table sugar, brown sugar, honey or dextrose.
Also, I’d recommend increasing the volume of the batch to 23L if you intend on adding the extra 375g of fresh ginger unless you’re keen for quite the ginger kick. You’ll need to add more sugar accordingly to maintain your alcohol content if you increase the volume.
Trizza.