i use 1 Liter ziploc bags and aquaclear tablets ( 1 tablet for a liter ) if i fulled it to the top i will know i got one liter <img src="images/graemlins/grin.gif" alt="" /> just throw in a tab per filled bag and i got the right dose. ( thinks of K.I.S.S. )
you could also mark the bags, full one liter in it and put a line of that water level ( on the outside ) and repeat it at the second liter level, etc. if you got a large bag...