I have not used drain opener for this and i dont know if it's the same as lye, but if it is, here's how you'd find the correct mix: put about a gallon of water in a bucket and put an egg in there. add some lye and observe the egg. look at the part above water- it should be about the size of a quarter. if its floating too high in the water, add lye. if it's floating too low, add water. adjust until the egg barely floats, with a quarter-sized bit above water.
or, go to your local wood-fired pizza place and ask for their ashes. remove the charcoal and use the fine white ash.