If you measure 20 ml of 50% citric acid and add 80 ml of water to it, you will end up with 100 ml of 10% citric acid .
If your mouth is in good condition this solution will keep it that way. If you have issues in your mouth and teeth you need something a little stronger.
A good mouthwash is made by adding 4 ml of Miracle-Mineral-Supplement in 500 ml of water. To use this take a mouthful and swish for 30 seconds, then spit it out. Activation is done inside your mouth and if you can swish for more than 30 seconds, go for it.
I add a drop of mint for a little "zest" from the mouthwash.