You really should dilute your Miracle-Mineral-Supplement down to a 5% solution and adjust the quantities used. We have had some problems handling the higher concentration solution, and the 5% is so much safer to handle. You can do everything with the 5% solution that you can with Miracle-Mineral-Supplement , you just have to adjust the amount used.
With that said, if you take the proper precautions in handling the Miracle-Mineral-Supplement solution, use 3 ml in 500 ml for the mouthwash. You will have to determine how many drops your set up makes per ml, if you want to measure in drops. I use a syringe or a graduated cylinder and measure directly.