This is my method:
Use an empty Dial brand foaming hand soap dispenser. I tried refilling Bath and Body’s but my method does not work with that dispenser.
2 tablespoons of Dawn Ultra dishwashing liquid. I use Mediterranean lavender scent. I have tried Palmolive in an effort to go extra cheap; but I was disappointed with the results)
3/4 cup of very warm water, to help the soap dissolve.
That is it. It is not exact science. I usually just eyeball it, but tonight I measured it out for a friend. I have thought of marking the spot that I fill it to with a permanent marker, but never have.