How do I make 25 volume developer?

I have a bottle of 20 vol. developer on hand, but I would like to lift my level only slightly more with a 25. I can't find it premixed ANYWHERE. I can't remember the formula. I've made a ten volume before using distilled water and a 20 vol dev. but simply cant remember how to do a 25 vol. I'm a cosmo student so I do have a basic knowledge of color theory but we didnt cover this part. I would like to hear from some pros.