I use 3 drops of glycerin to 1.5 oz. of 91% alcohol

Other recommend different percentages. You can play it to see what you like IE
2 drops of glycerin to 1.5 oz of alcohol
3 drops of glycerin to 2 oz of alcohol
4 drops of glycerin to 1.5 oz of alcohol

The glycerin is vegetable glycerin. I got my first one at a health food store. Then I bought some on Amazon.

Note: I have found sometimes on the tile the inks rub off really easily when dried. Perhaps if they are sealed this would not happen.