One ml has the mass of 1 gram...
9ml of PG + 1 gram of EM crystals = 10 grams. If 1 gram of the total 10 gram solution is EM crystals, 1/10, 10%...
If you dont have a scale, the 1/4 teaspoon in 8/9/10 ml of PG is safe.
The Flavor Apprentice says use 8, science says 9, and ECX says 10...