knthrak1982 said:
Very roughly speaking, for a 5 gallon batch, 1kg of dry malt extract will give you an extra 1.5%. Add hops to balance the sweetness. For sugar, it's more like 2.5% if you add a whole kilogram but it won't be good for flavour. You need a yeast that can handle high gravity/alcohol, possibly using one yeast after another. Look at people's Belgian Trippel brew days for example.
I think at least 75% of most DME is fermentable, so the (still rough) figure for 1Kg of that in 23L would be closer to 1.9%ABV than 1.5%.
Unless you want a dramatic increase in ABV, I would just brew a little short (by 2 or 3 litres on a 23L kit) and add a bit more sugar (maybe 500g). At least try that once and see if you like the result.
I think quite a lot of homebrewers ignore the effect of priming sugars on ABV too and quote their results without. Using the forum calculator alone (OG to FG) will not account for that, so if you're really keen to calculate the final, packaged, ABV as close as possible, include the effect of the priming sugars. This formula will get it as close as I know how, without actually taking the gravity of a dissolved priming solution:
(Batch size being packaged in litres / Fermentable priming sugar in grammes)/18
So 80g table sugar in 23L will add roughly 0.2% ABV. For some of my fizzy Belgian ales, I've added enough to raise the ABV by nearly 0.5%.
The HMRC calculations are
here (click), which are used by small breweries without a full lab.
You may be slightly closer to your target than you realise!