adding lactose at bottling/kegging time is ideal to decide on how much sweetness you want to add, you can take a very small sample and add some, then if you like it you can multiple it out to the whole batch.. i was going to do that to my choc stout i have kegged now but SWMBO vetoed the use of lactose.. so i can't call it a "Milk Choc Stout" anymore :-( sigh..