Boost the strength? Mine came out bang on 6% so with conditioning you're looking at 6.2%. Was in primary for 2 then secondary for 2. Bottled warm for 2 then a few in the fridge. Very drinkable even after a month of bottling. I guess if you want to push the abv higher the usual trick is to add less water, so brew to 21litres. That should add approx 0.5%, or you could add more sugar, but it tends to make the brew leaner.