my water treatment says "to remove chlorine" from tap water add 1 ml per 75.7 litres of water, and to detoxify chloramines add 1 ml per 19 litres of water...
I had the bran recomended to me by a friend so while my tank is cycleing i stocked up on lots of there meds, (melafix, pimafix, stress coat, stress zyme,) and saw the water conditioner so thought id try that aswell...
Best thing to do is to contact your water company, they will mail you a detailed water analysis. Second best thing to do is to ask a reliable lfs who is on the same water supply as you if it contains chloramine.
Chloramine is a compound made of chlorine & ammonia. Nearly every dechlorinator will get rid of chloramine, the problem with some is that they don't get rid of the ammonia. If you have chloramine, make sure you get a water treatment that neuteralizes ammonia as well.
I am currently building a database of UK water companies chloramination policies and may well be able to give you the answer (it's about 75% complete at the moment).
How they can tell you how much to use when chloramine concentration ranges from <0.1 mg/l to >1 mg/l, I do not know.