I don't rinse. I simply put in as much as needed and continue to march on. When it is time to renew, I let it drain and then give it a toss into the garbage. There is a bio-degradable version that is landfill safe and cost only a fraction more. When it is on sale I snatch up as much as possible.
You will need a few tries to establish what is too little or too much. I think you will be as happy with it as we are.