In October 2012, Microsoft released a new version of Microsoft Office Mobile for Windows Phone 8 and Windows Phone 7.8. This was followed up by the release of Office Mobile for iPhone on 14 June 2013, and Office Mobile for Android phones on 31 July 2013.
That's an easy number to work with, so let's use it. 20 drops/milliliter = 20,000 drops/liter =1,000,000 drops / 50 liters. That means one part per million is the same as one drop of a substance in a million drops, or 50 liters, of water.
However, most of the good dechlorinators - that don't do a lot of unnecessary extra stuff - are dosed between one drop for each two gallons of water an two drops for each gallon of water (that's between about 1 drop for each 8 liters and one drop for each 2 liters).