This question sounds more like DIY than maths!
If you want to add a certain pertain percentage of water to putty, then I imagine that it should be done by weight, most probably. 15% of 250 gm is 10% + 5% = 25+12.5 = 37.5 gm
And 1 ml of water is 1 gm. So 37.5 gm water is the same as 37.5 ml water.
37.5 ml is about 2 1/2 tablespoons or 1 1/4 fluid ozs.
When adding water to putty, you should really only add it a bit at a time and keep stirring/mixing until it reaches the required (i.e. workable) consistency.
Best of luck!