Yep I have noticed the long duration pulls have benefitted immensely with the water, as far as keeping the iat low and the timing stable.
I do have a hygrometer but it's for home brew, not fuel mixing. I'll just mix it by volume, it's good enough, not that crucial and the knock control is in the tune rather than the addition of the water meth. The fuel can change up or down a point, I've got it very rich anyway cus it still has stock turbo pistons. For example, I could switch the water off and the current tune will be fine, it just pulls all the timing after a long run.