This form calculates how much water to add to a glaze to obtain a desired specific gravity. https://pietermostert.github.io/SG_calc/html/specific-gravity-1.html
If your bucket of glaze is too heavy to weigh, use this instead: https://pietermostert.github.io/SG_calc/html/specific-gravity-2.html
Based on calculator by Jason Bruce: https://github.com/jbruce12000/glaze-specific-gravity-calculator