Was having the same problem this morning with a sulfa powder, but I figured out a way to do it.
Take your package of powder, dump it all in a clean dry bowl. Measure it all back into the package by the teaspoon. Write down how many teaspoons you have.
The package, if it's like mine, will tell you how many gallons or liters the whole package should be mixed with.
Now you can take your number of teaspoons in the total package, divide it by the number of gallons/liters of water the package is supposed to make, and that will give you your appropriate fraction of a teaspoon needed for one gallon. Then divide the fraction further if you're making less.
Hope this helps.