.5 teaspoon (recommended amount per 500ml) is about 1.45g. That means that 50 x 500ml x 1.45g = 72g. This seems a little short to me. I use BeerSmith to work through all my calculations and it seems to arrive at approximately 130g per 25ltr brew. So there you go - somewhere between 72 and 130g should do it. If it were me I'd split the difference and go for somewhere around 100g.
I think converting volume to weight (especially with sugar) is far from an exact science. One report I checked listed a teaspoon of sugar (as teaspoons of different things will weigh different amounts) as 4.2g - therefore .5 would be 2.1g...
So 12 ltrs would either need 34.8g or 50.4.
I'd suggest splitting the difference
