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.