I think you need to actually be able to calculate the mass of the extract, not the weight. Basically the density of the extract determines the mass/volume ratio:
Density=mass/volume
So if the starting gravity of the extract is 1.100, then 1.100 = mass/1L, then you would need 1.1 kg per liter of extract (or vice versa). If you can't find the mass, then weight is probably a decent estimate.
I'm estimating that starting gravity for the extract, but if thats the case, then thinking back to my extract days, 6 lbs = 2.72 kg would mean approx. 2.5 liters of extract, which sounds about right.
I could be wrong, but that's how I figure it.