Thank you very much. Your logic is right, but I wanted to make different amount of colored slips and the mathematical approach is the following:
I made 500 g dry "slip" and adding water I ended up with 860 ml wet slip.
Dividing 500/860=0.58g dry material in every 1 ml wet slip.
In 300g wet slip I have 300x0.58=174g dry material . To add 10%mason stain for black slip I need (174/100)x10=17.4g stain
1. Find how much dry material you have in 1ml wet slip -by dividing dry on wet.(500/860)=0.58
2. Multiply the wet amount you want (300)with the dry in 1 ml to have the dry amount into it(300x0.58)=174g dry
3. Calculate the percentage of stain necessary for the dry(174) material used for the wet slip.
This way you can make different amounts of stained slip.