I have read a lot of threads here that describe amounts of a chemical required to make a working solution (or stock) using a concentrate that often requires mental gymnastics to work out how much of your concentrate are needed to make up a 'brew'. A trick I learned over 40 y ago in my line of business (anesthesiology) was a quck way to work out how much of the agent, in our case an amount of say Phenidone, you would add of a percentage solution you have to deliver the milligrams you want in the mix. The 'rule de thumb' is you mentally multiply the percentage by ten and call it mg/ml. E.G. a 1% (1 gram in 100ml of glycol) has (1 x 10) mgm / ml of phenidone. 10mgm. A 10% solution has 10 x 10 = 100 mgm/ml of the agent. A 2.5% solution has 2.5 x 10 = 25 mgm/ml of phenidone. And so it goes. If this is a puzzle to you, forget I said it! It can speed up ad hoc mixing of developers etc. if you remember the multiply by ten 'rule' Hope this is of some help to hombrewers. Murray Brisbane, Oz.