I talked for an hour at the NHC about bottle conditioning...the amount of sugar to add is the critical part! The standard amount of 2/3 cup corn sugar (aka dextrose) for 5 gallons usually works; if you only have 2.5 gallons, you would use 1/3 cup. The amount of water you dissolve in is not too important--but it should be boiled to sterilize it (and cooled before adding to the bottling bucket). If your beer finishes dryer, you'd need to add extra sugar; if it finishes sweeter (as measured with a hydrometer), add less sugar. Commercial breweries dial in the sugar dosing with great accuracy; there's a way to try three or more sugar dosing concentrations in a 5 gallon batch.