Your gravity is diluted by the ratio of old volume to new volume. So: New Gravity Points = 58 x (5.25 / 5.75) = 52.9 So your new gravity = 1.0529 or 1.053.
The confusing thing about gravity math is the "1.xxx" you have to deal with, which throws simple math out the window. For example, if you multiply 1.058 by your volume ratio, the answer is wrong. Easier just to work with the Gravity Points, which is why the equation uses 58 instead of 1.058.